About: Adobe Acrobat Reader DC software is the free global standard for reliably viewing, printing, and commenting on PDF documents. And now, it's connected. Adobe Acrobat is a family of application software and Web services developed by Adobe Inc. to view, create, manipulate, print and manage files in Portable Document Format (PDF). The family comprises Acrobat Reader (formerly Reader ), Acrobat (formerly. The Portable Document Format (PDF) is a file format developed by Adobe in the s to PDF was standardized as an open format, ISO , in , and no longer requires any royalties for its implementation. Today, PDF files may.
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Invented by Adobe Inc. and perfected over more than 25 years, Portable Document Format (PDF) is an open standard for electronic document exchange. Download free Acrobat Reader DC software, the only PDF viewer that lets you read for reliably viewing, printing, signing, and commenting on PDF documents . Adobe Acrobat Reader is the most reliable, free global standard document management system available. View, edit, sign, and annotate PDF documents by .
Three letters that changed the world. What is PDF? Portable Document Format PDF is a file format used to present and exchange documents reliably, independent of software, hardware, or operating system. PDFs can contain links and buttons, form fields, audio, video, and business logic. They can also be signed electronically and are easily viewed using free Acrobat Reader DC software. We invented PDF.
The mechanism is similar to those that are sometimes used to prevent unauthorized copying of software to other computers.
These security settings mean that authors can choose to limit who uses their documents and how. The Adobe Reader program ensures that a PDF file can be viewed with the same visual fidelity on almost any type of computer. Since these programs may be obtained without charge, the cost of the Adobe Reader software is not an obstacle to viewing a document that is available in PDF. This cross-platform portability means that authors can disseminate their works widely.
The popularity of PDF as a means of distributing publications has some benefit for people who are blind or have impaired vision. In general, electronic publications offer more potential for accessible, independent reading than do print publications, since computer programs can produce output in flexible and alternative ways, including synthetic speech, braille, and magnified text.
This means that an intermediary sighted assistant is not needed, thus providing convenience and privacy. The benefits of PDF, previously discussed, help to increase the amount of reading material that is published in electronic form. In addition, someone who is visually impaired benefits directly, as others do, from particular PDF features, such as compact storage. Yet, some PDF features that provide benefits of a general nature have had inadvertent adverse side effects for nonvisual readers.
To understand why, this section explains some technical inner workings of PDF. The specification for the current version 1.
To keep within the scope of this article, the discussion will necessarily simplify a technical explanation of the format, focusing on the concepts most relevant to accessibility. PDF originates in a specialized programming language, called PostScript, developed by Adobe in the s. Part of the power of PostScript derives from its flexibility about the order in which parts of output are placed on a page.
The order does not have to be from left to right and top to bottom. A PostScript-enabled printer produces output a page at a time. Each page of output is transmitted as a batch after all drawing operations on it are complete. An observer of the visual page may guess, but does not actually know, in what order the output was drawn. Producing output may be subdivided into drawing three components: How these different objects are used and combined has implications for accessibility, as explained later.
Textual characters are based on a font table: Unicode, by comparison, defines tens of thousands of characters in order to support numerous written languages of the world, as well as many specialized symbols used in particular subject areas. A PostScript program draws a string of characters on a page by using the Unicode value of each character and looking up its associated shape in a font table. Besides textual characters, many other kinds of shapes may be drawn on a page based on mathematical calculations.
Such shapes--called vector graphics--may be straight or curved lines, geometric designs such as circles or squares, or filled areas according to a pattern. In fact, PostScript can draw vector graphics to create a picture of almost anything on a page. A third component of output is a photographic image, which may be thought of as an array of colored dots that create a literal picture.
PostScript does not know the internal structure of an image, so it essentially copies rather than generates it to a particular location on the page. PDF is a way that documents can be viewed on the screen and exchanged among users, not just printed onto paper. PDF uses the same "imaging model" as PostScript for describing how a page looks.
The commands and data follow certain rules that Adobe has defined as the specification for Portable Document Format. As opposed to a file format whose internal structure is only known by its developers, the PDF specification is published and open rather than private and proprietary. It is copyrighted and controlled by Adobe, but anyone is free to use it for developing software that either creates or views PDF files within general licensing terms.
Adobe also publishes a free viewing and printing program for many different devices so that all understand PDF in the same way. Adobe has, therefore, established the combination of a file format and software interpreter that enables authors to publish documents with a certain look and feel for potential readers in a broad variety of environments.
PDF files may be subdivided into three types: These types differ in their use of the different components just described--textual characters, vector graphics, and photographic images.
An image-only PDF contains a photographic image representing each page, and virtually no textual characters or vector graphics. Although text may appear on a page, the text is actually a surface picture without underlying characters. Individual characters are needed for translation into speech or braille, so an image-only PDF file is inaccessible.
Image-only PDF files are usually created by scanning hard-copy documents into a computer with attached scanning equipment. Essentially, the system takes a picture of each printed page and then packages the pages in a PDF file. It is possible to use optical character recognition OCR software to create textual characters in the PDF file, but this is often not done because the process takes much longer: Another reason for avoiding OCR is that the resulting text usually contains recognition errors that require manual proofreading and correction to be accurate, thereby involving more staff time and skill.
Scanning documents into image-only PDF files has been a common way of storing information for archival purposes because electronic media are much smaller and less cumbersome than is paper storage. The more that documents originate in electronic, rather than hard-copy, form, the less likely that documents need to be scanned to be archived. Thus, as authors rely more on computers as the original source of documents, the accessibility problem of image-based PDF may lessen over time.
Searchable-image PDF also contains an image for each page, but this type includes a text layer as well. The textual characters are produced from an OCR process, which analyzes each image for what appear to be characters. Wherever characters are recognized in the image, the software draws a layer of text under them. An observer of the page sees the surface image only, as with image-only PDF.
The text layer enables a PDF file to be searched for phrases of interest to a reader who is viewing the document. This text also enables PDF files to be indexed with keywords in a collection of electronic documents, thus permitting a researcher to find particular ones worth further study. Adding a text layer increases the size of a PDF file, so text may be omitted if compactness is of primary importance. Usually, however, the ability to search, for sighted as well as visually impaired readers, outweighs the cost in extra size, especially since the text is compressed, as previously mentioned.
A third PDF type, called formatted text and graphics, minimizes the use of photographic images in favor of textual characters and vector graphics. No image layer rests on top of a text layer. Instead, textual characters and vector graphics are drawn wherever they can represent the content of a page. Photographic images are used only when they are pictures that cannot be generated from building blocks of textual characters and vector graphics. This type of PDF is usually the result of conversion from another electronic file format, such as Microsoft Word.
Also, since this type is built from more structured components, it may be used more flexibly for other purposes. A PDF file composed as formatted text and graphics is likely to be more accessible than one composed as searchable image. Although both types contain textual characters, the quality of the text is almost necessarily better in the latter type because it serves the purpose of presentation as well as searchability. If the PDF file was created by scanning, more work has probably been done than with the searchable-image type in order to correct OCR errors and achieve presentable text.
If the PDF file was created by converting another electronic format, then the textual components are probably more complete, since they derive directly from character fonts rather than indirectly from recognized images. Despite the accessibility potential of this PDF type, however, other problems of a structural nature may pose significant accessibility problems, as subsequently explained.
Textual characters are a necessary condition for the accessibility of PDF, but they are not sufficient on their own. Some PDF-creation tools do not leave enough information about the fonts used for a PDF viewing program to decipher all the characters in terms of a well-understood computer alphabet. The viewing program sees shapes that it knows are characters drawn on the page.
The program then has to do a back-translation of their drawing operations, looking up the Unicode value for each shape and rendering it as a standard screen character. If the original font table is embedded in the PDF file, the viewing program can decode the characters. Decoding is also possible if a common font was used, such as one built into the operating system. Without an available font table, however, the viewing program does not know what textual characters exist because it does quick table lookups rather than sophisticated OCR.
Even if complete character decoding is possible, a PDF file may be inaccessible because of problems in "reading order. Can they be extracted from the text of the PDF file in a coherent, linear order, or are they mixed together in disconnected, confusing ways?
For example, the text of a PDF file may appear visually like newspaper columns, where a line stops midway across the page and continues underneath, rather than continuing across to the right margin. Visually, on a screen or printout, the structure of the document is apparent because of extra spacing or a border line that indicates where one column of text ends and another begins. Information about this document structure, however, must be represented in the PDF file for the reading order to be rendered in an intelligible manner by assistive technology.
Without structural information that groups and separates regions of the page, the document may be inaccessible to nonvisual readers. Since PDF is frequently chosen for publications that are intended to look fancier than single-column text, PDF files often contain irregular page layouts with multiple columns, sidebars, and picture captions.
If these files lack an internal structure, a nonvisual interpretation of them necessarily involves guesses about reading order, and mistakes can seriously undermine the comprehension of their content.
To address such accessibility problems, Adobe introduced an extension to PDF called "tagging. HTML encloses portions of text with markers that indicate the structure or purpose of the text. For example, a phrase may be tagged as the heading of a section, the caption of an image, or a cell within a table. Some tags are necessary for proper visual display in a web browser that interprets HTML files, whereas other tags--although still a standard part of the HTML language--are recommended specifically to aid accessibility.
For example, accessibility tags include an indication of the row and column labels of a table, which enables a screen reader to tell the user about the context of each cell. The cell information may be useless or confusing without knowing the associated row and column labels.
Collectively, the HTML tags that are needed for accessibility are sometimes called "accessible markup. The tagged PDF that Adobe developed provides similar functionality. Tags mark portions of PDF content and are organized in a sequence that conveys the suggested reading order. Whereas HTML files are readable text with tags as words enclosed in brackets, however, PDF files are in a compressed, binary form with tags that can be viewed only with special software, such as Adobe Acrobat.
The U. See For More Information at the end of this article for a link to these regulations. Section mandates that federal agencies provide information to people with disabilities in a manner that is comparable to that provided to people without disabilities.
Section does not require software manufacturers to make accessible products, but it does provide them with significant market incentives to do so because the federal government is a large customer that is interested in products that meet minimum standards of accessibility.
Indeed, Congress adopted Section partly with the stated purpose of creating voluntary market incentives to develop technologies that benefit people across a broad range of physical characteristics, not just those with typical levels of eyesight, hearing, manual dexterity, and other traits. Adobe, like other companies that sell to the federal government, has noticeably increased the accessibility of its products in recent years, and its web site includes information on compliance with Section standards.
The tagged PDF format is an accessibility innovation that the company introduced in The program is available in both a Standard and Professional version, with the latter having the most tagging features and being recommended by Adobe to customers who are concerned with accessibility.
The basic content and layout of a PDF document is usually created and revised using a word-processing program, such as Microsoft Word or Corel WordPerfect, and is then converted to PDF to create the final form, exploiting features like visual fidelity, compact storage, security settings, and cross-platform portability, as previously described.
It lets one combine multiple source documents into a single PDF file, such as a report consisting of a Microsoft Word narrative and a Microsoft Excel spreadsheet. Layers, or as they are more formally known Optional Content Groups OCGs , refer to sections of content in a PDF document that can be selectively viewed or hidden by document authors or consumers.
This capability is useful in CAD drawings, layered artwork, maps, multi-language documents etc. Basically, it consists of an Optional Content Properties Dictionary added to the document root. This dictionary contains an array of Optional Content Groups OCGs , each describing a set of information and each of which may be individually displayed or suppressed, plus a set of Optional Content Configuration Dictionaries, which give the status Displayed or Suppressed of the given OCGs.
Security and signatures[ edit ] A PDF file may be encrypted for security, or digitally signed for authentication. The user password encrypts the file, while the owner password does not, instead relying on client software to respect these restrictions. An owner password can easily be removed by software, including some free online services.
Even without removing the password, most freeware or open source PDF readers ignore the permission "protections" and allow the user to print or make copy of excerpts of the text as if the document were not limited by password protection. Not only can they restrict document access but they also reliably enforce permissions in ways that the standard security handler does not. The signature is used to validate that the permissions have been granted by a bona fide granting authority.
Adobe Reader verifies that the signature uses a certificate from an Adobe-authorized certificate authority. Any PDF application can use this same mechanism for its own purposes.
Metadata[ edit ] PDF files can contain two types of metadata. This is stored in the optional Info trailer of the file. A small set of fields is defined, and can be extended with additional text values if required. This method is deprecated in PDF 2. In PDF 1. This allows metadata to be attached to any stream in the document, such as information about embedded illustrations, as well as the whole document attaching to the document catalog , using an extensible schema.
Usage restrictions and monitoring[ edit ] PDFs may be encrypted so that a password is needed to view or edit the contents.
PDF 2. PDF files may also contain embedded DRM restrictions that provide further controls that limit copying, editing or printing. These restrictions depend on the reader software to obey them, so the security they provide is limited. Default display settings[ edit ] PDF documents can contain display settings, including the page display layout and zoom level. Adobe Reader uses these settings to override the user's default settings when opening the document.
Intellectual property[ edit ] Anyone may create applications that can read and write PDF files without having to pay royalties to Adobe Systems ; Adobe holds patents to PDF, but licenses them for royalty-free use in developing software complying with its PDF specification. Some software can automatically produce tagged PDFs, but this feature is not always enabled by default.