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"PDF" redirects here. For other uses, see PDF (disambiguation).
Portable Document Format (PDF)
File:PDF.png
File:Acroread.png
Adobe Reader displaying a PDF in Microsoft Windows Vista
Filename extension
.pdf
Internet media type
application/pdf
Type code'PDF ' (including a single space)
Uniform Type Identifier (UTI)com.adobe.pdf
Magic number%PDF
Developed byAdobe Systems

Portable Document Format (PDF), sometimes mistaken for "Printable Document Format", is an open file format created by Adobe Systems in 1993 and is now being prepared for submission as an ISO standard.[1] It is used for representing two-dimensional documents in a device independent and resolution independent fixed-layout document format. Each PDF file encapsulates a complete description of a 2D document (and, with the advent of Acrobat 3D, embedded 3D documents) that includes the text, fonts, images, and 2D vector graphics that compose the document.

While PDF can describe very simple one page documents, it may also be used for many pages, complex documents that use a variety of fonts, graphics, colors, and images.

History

When PDF first came out in the early 1990s, it was slow to catch on[citation needed]. At the time, not only did the only PDF creation tools of the time (Acrobat) cost money, but so did the software to view and print PDF files. Early versions of PDF had no support for external hyperlinks, reducing its usefulness on the web. Additionally, there were competing formats such as Envoy, Common Ground Digital Paper and even Adobe's own PostScript format (.ps). Adobe soon started distributing the Acrobat Reader (now Adobe Reader) program at no cost, and continued to support PDF through its slow multi-year ramp-up. PDF eventually became the de facto standard for printable documents.

The PDF file format has undergone several changes, generally as new versions of Adobe Acrobat have been released. There have been eight versions of PDF: 1.0 (1993), 1.1 (1994), 1.2 (1996), 1.3 (1999), 1.4 (2001), 1.5 (2003), 1.6 (2005), and 1.7 (2006). These correspond to Acrobat releases 1.0 to 8.0.

Technology

Anyone may create applications that read and write PDF files without having to pay royalties to Adobe Systems; Adobe holds a number of patents relating to PDF but licenses them royalty-free for use in developing software that complies with its PDF specification.[2]

PDF is primarily the combination of three technologies:

  • A sub-set of the PostScript page description programming language, for generating the layout and graphics.
  • A font-embedding/replacement system to allow fonts to travel with the documents.
  • A structured storage system to bundle these elements and any associated content into a single file, with data compression where appropriate.

PostScript

Main article: PostScript

PostScript is a page description language that is run in an interpreter to generate an image. This process requires a fair amount of resources.

PDF is a file format instead of a programming language. For instance, flow control commands like if and loop are removed, while graphics commands such as lineto remain.

Often, the PostScript-like PDF code is generated from a source PostScript file. The graphics commands that are output by the PostScript code are collected and tokenized; any files, graphics or fonts the document references are also collected; and finally everything is compressed into a single file. Therefore, the entire PostScript world in terms of fonts, layout and measurement remains intact.

As a document format, PDF has several advantages over PostScript:

  • Already tokenized: PDF contains already-tokenized and interpreted results of the PostScript source code, so there is a more direct correspondence between changes to items in the PDF page description and changes to the resulting appearance of the page.
  • Object transparency: PDF (starting from version 1.4) supports true transparency while PostScript does not.
  • Independent pages: PostScript is an imperative programming language (with an implicit global state), so instructions accompanying the description of one page can affect the appearance of any following page. All the preceding pages must therefore be processed in order to determine the correct appearance of any given page. Each page in a PDF document is unaffected by any others.

Accessibility

PDF files that are accessible to people with disabilities can be created. Current PDF file formats can include tags (essentially XML), text equivalents, captions and audio descriptions, and other accessibility features. Some software, such as Adobe InDesign, can output tagged PDFs automatically. Leading screen readers, including JAWS, Window-Eyes, and Hal, can read tagged PDFs; current versions of the Acrobat and Acrobat Reader programs can also read PDFs out loud. Moreover, tagged PDFs can be reflowed and zoomed for low-vision readers.

However, many problems remain, not least of which is the difficulty in adding tags to existing or "legacy" PDFs; for example, if PDFs are generated from scanned documents, accessibility tags and reflowing are unavailable and must be created either by hand or using OCR techniques. Also, these processes themselves are often inaccessible to some people with disabilities. Nonetheless, well-made PDFs can be a valid choice as long-term accessible documents. PDF/UA, the PDF/Universal Accessibility Committee, an activity of AIIM, is working on a specification for PDF accessibility based on the PDF 1.6 specification.

One of the major problems with PDF accessibility is the fact that PDF documents have 3 distinct views which, depending on how the document was created, can be inconsistent with each other. The three views are the physical view, the tags view and the content view. The physical view is what gets displayed and printed and is what most people think of as the PDF document. The tags view is what screen readers read and is useful for people with visual disabilities. The content view is what gets displayed when the document is reflowed in Acrobat and is useful for people with mobility disabilities. In order for a PDF document to be accessible, the three views have to be consistent with each other.

Usage restrictions and monitoring

PDFs may be encrypted so that a password is needed to view or edit the contents. The PDF Reference defines both 40-bit and 128-bit encryption, both making use of a complex system of RC4 and MD5. The PDF Reference also defines ways in which third parties can define their own encryption systems for use in PDF.

PDF files may also contain embedded DRM restrictions that provide further controls that limit copying, editing or printing. The restrictions on copying, editing, or printing depend on the reader software to obey them, so the security they provide is limited. Printable documents especially might be saved instead as bitmaps and subject to OCR.

The PDF Reference has technical details or see [1] for an end-user overview. Like HTML files, PDF files may submit information to a web server. This could be used to track the IP address of the client PC, a process known as phoning home.

Through their LiveCycle Policy Server product, Adobe provides a method to set security policies on specific documents. This can include requiring a user to authenticate and limiting the time frame a document can be accessed or amount of time a document can be opened while offline. Once a PDF document is tied to a policy server and a specific policy, that policy can be changed or revoked by the owner. This controls documents that are otherwise "in the wild." Each document open and close event can also be tracked by the policy server. Policy servers can be set up privately or Adobe offers a public service through Adobe Online Services.

Subsets

Proper subsets of PDF have been, or are being, standardized under ISO for several constituencies:

  • PDF/X for the printing and graphic arts as ISO 15930 (working in ISO TC130)
  • PDF/A for archiving in corporate/government/library/etc environments as ISO 19005 (work done in ISO TC171)
  • PDF/E for exchange of engineering drawings (work done in ISO TC171)
  • PDF/UA for universally accessible PDF files

A PDF/H variant (PDF for Healthcare) is being developed.[3] However, it may consist more in a set of "best practices" than in a specific format or subset.

Mars

According to a 7 December 2006 Government Computer News blog, Joab Jackson writes that Adobe is exploring an XML-based next-generation PDF codenamed Mars: http://www.gcn.com/blogs/tech/42740.html

Adobe has published information about the Mars file format at http://www.adobe.com/go/mars and at http://labs.adobe.com/wiki/index.php/Mars.

Content

A PDF file is often a combination of vector graphics, text, and raster graphics. The basic types of content in a PDF are:

  • text stored as such
  • vector graphics for illustrations and designs that consist of shapes and lines
  • raster graphics for photographs and other types of image

In later PDF revisions, a PDF document can also support links (inside document or web page), forms, JavaScript (initially available as plugin for Acrobat 3.0), or any other types of embedded contents that can be handled using plug-ins.

PDF 1.6 supports 3D documents embedded in the PDF that can be interacted with.

Two PDF files which look similar on a computer screen may be of very different sizes. For example, a high resolution raster image takes more space than a low resolution one. Typically higher resolution is needed for printing documents than for displaying them on screen. Other things that may increase the size of a file is embedding full fonts, especially for Asiatic scripts, and storing text as graphics.

Base 14 Fonts

There are fourteen typefaces that have a special significance to PDF documents: Times Roman (in standard, italic, bold, and bold oblique), Courier (in standard, oblique, bold and bold oblique), Helvetica (in standard, oblique, bold and bold oblique), Symbol and Zapf Dingbats. These should always be present (actually present or a close substitute) and so need not be embedded in a PDF. http://www.planetpdf.com/planetpdf/pdfs/pdf2k/03e/merz_fontaquarium.pdf] PDF viewers must know about the metrics of these fonts. Other fonts may be substituted if they are not embedded in a PDF.

Implementations

Readers for many platforms are available, such as Xpdf, Foxit, and Adobe Reader; there are also front-ends for many platforms to Ghostscript. PDF readers are generally free. There are many software options for creating PDFs, including the PDF printing capability built in to Mac OS X, the multi-platform OpenOffice.org, Microsoft Office 2007 (an additional free download from Microsoft is required), numerous PDF print drivers for Microsoft Windows, and Adobe Acrobat itself. There is also specialized software for editing PDF files.

PDF was selected as the "native" metafile format for Mac OS X, replacing the PICT format of the earlier Mac OS. The imaging model of the Quartz graphics layer of Mac OS X is based on the model common to Display PostScript and PDF, leading to the nickname "Display PDF". System-level support for PDF allows Mac OS X applications to create PDF documents automatically, provided they support the Print command. When taking a screenshot under Mac OS X versions 10.0-10.3, the image was also captured as a PDF; in 10.4 the default behaviour is set to capture as a PNG file, though this behaviour can be set back to PDF if required.

Some printers also support direct PDF printing, which can interpret PDF data without external help. Currently, all PDF capable printers also support PostScript, but most PostScript printers do not support direct PDF printing.

Advantages

Electronic documents based on PDF have countless advantages over traditional archiving formats (e.g. paper or microfilm). Improved accessibility alone may substantiate the implementation of an electronic archive. Some advantages of a PDF archive over a TIFF or a paper-based archive are:

  • PDF stores objects (e.g. text, graphics), allowing for an efficient full-text search in an entire archive. TIFF is a raster format and must first be scanned with an OCR (optical character recognition) engine.
  • PDF files can require only a fraction of the memory space of original or TIFF files[citation needed], without loss of quality. The smaller file size is especially advantageous for electronic file transfers (FTP, e-mail attachment etc.)
  • PDF format can be optimized. The optimization can be focused on images (e.g. scanned checks) or extracting structured data (e.g. voucher information). TIFF treats all file information the same.

References

  1. ^ Adobe Systems Inc (29 January 2007). "Adobe to Release PDF for Industry Standardization". Retrieved 2007-01-31. {{cite web}}: Check date values in: |date= (help)
  2. ^ http://partners.adobe.com/public/developer/support/topic_legal_notices.html
  3. ^ AIIM (2006-10-20). "New Best Practices Guide Addresses Exchange of Healthcare Information". Retrieved 2007-03-09.

See also

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