Portable Document Format

PDF redirects here. For other uses, see PDF (disambiguation).

Portable Document Format (PDF) is a file format developed by Adobe Systems for representing documents in a manner that is independent of the original application software, hardware, and operating system used to create those documents. A PDF file can describe documents containing any combination of text, graphics, and images in a device independent and resolution independent format. These documents can be one page or thousands of pages, very simple or extremely complex with a rich use of fonts, graphics, colour, and images. PDF is an open standard, and anyone may write applications that can read or write PDFs royalty-free.

Free readers for many platforms are available for download from the Adobe website [1], and there are several free open source readers, including Xpdf [2] for POSIX-like systems with the X Window System; GPdf [3], a derivative of Xpdf for GNOME, Evince [4], a document viewer for GNOME (branch of GPdf) that can view PDF-files; GSPdf [5] and ViewPDF [6], for GNUstep; and front-ends for many platforms to Ghostscript.

Contents
1 Technology

1.1 PostScript
1.2 History
1.3 Macintosh

2 PDF and accessibility
3 Comparison between PDF and HTML
4 Searching for a text in a collection of files
5 Types of content
6 See also
7 References
8 External links

8.1 Tools

8.1.1 Commercial PDF Software
8.1.2 Free and open source software

8.1.2.1 Creator
8.1.2.2 Viewer
8.1.2.3 Editor

8.1.3 Closed source software

Technology

PDF is primarily the combination of three technologies:

PostScript

PostScript is a computer language – or more precisely 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 subset of those PostScript language elements that define the graphics, and only requires a very simple interpreter. For instance, flow control commands like if and loop are removed, while graphics commands such as lineto remain.

That means that the process of turning PDF back into a graphic is a matter of simply reading the description, rather than running a program in the PostScript interpreter. However the entire PostScript world in terms of fonts, layout and measurement remains intact.

Often the PostScript-like PDF code is generated from a source PostScript file. The graphics commands that the PS code outputs are collected and tokenized, any files, graphics or fonts the document references are also collected, and then everything is compressed into a single file.

As a document format, PDF has several advantages over PostScript. One is that a document resides in a single file, whereas the same document in PostScript may span multiple files (graphics, etc.) and probably occupies more space. In addition, PDF contains already-interpreted results of the PostScript source code, so it is less computation-intensive and faster to open, and there is a more direct correspondence between changes to items in the PDF page description and changes to the resulting appearance of the page. Also, PDF (starting from version 1.4) supports true object transparency while PostScript does not. Finally, if displayed with Adobe Reader, a font-substitution strategy ensures the document will be readable even if the end-user does not have the "proper" fonts installed.

History

When PDF first came out, in the early 1990s, it was slow to catch on. 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. Additionally, there were competing formats such as Envoy, Common Ground Digital Paper, DjVu and even Adobe's own PostScript file format (.ps). Adobe started distributing the Acrobat Reader program at no cost, and continued to support PDF through its slow multi-year ramp-up. Competing formats eventually died out, and PDF became a well-accepted standard.

In 2005 Microsoft presented a competing format referenced by the code name "Metro". It is developed together with Global Graphics. Metro is based on XML, but requires a license. Metro is scheduled to be included in the next version of MS Windows with the codename "Longhorn".

Macintosh

PDF was selected as the "native" metafile format for Mac OS X, replacing the PICT format of the earlier Mac OS. Mac OS X's imaging model, Quartz 2D, is based on both the Display PostScript standard and PDF, and is sometimes referred to as Display PDF. Due to OS support, all OS X applications can create PDF documents automatically as long as they support the Print command.

PDF and accessibility

PDF can be accessible to people with disabilities. 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. Moreover, that process itself is inaccessible. Nonetheless, well-made PDFs can be a valid choice as long-term accessible documents. (Work is being done on a PDF variant based on PDF 1.4. The PDF/A or PDF-Archive is specifically scaled down for archival purposes.)

Comparison between PDF and HTML

PDF and HTML are not equivalent technologies, but are both commonly found on the Web. Using the reflow feature, tagged PDF documents behave much like HTML.

HTML is a method for describing the content of a webpage in a manner that is open to interpretation by the browser which renders it on the user's screen. This permits content to be rendered to suit the viewer rather than the content provider, and also means that an HTML file will not necessarily look exactly the same in different browsers. PDF without applying reflow, on the other hand, is strictly concerned with describing the content of a document such that the original layout and typesetting are fully preserved.

Since many content providers do not like the fluid nature of HTML rendering, PDF without reflow has been widespread to force a particular layout. With HTML the same can be achieved by using a raster graphics (or recently, SVG, a vector graphics standard) image to present text, but then the text can not be copied as such, nor can a subtext be searched within it. Use of images also leads to larger file sizes. (Sometimes the same is done in a PDF file, and the same disadvantages apply.)

A typical example of the differences this leads to for the viewer is with zooming:

Searching for a text in a collection of files

Adobe Acrobat Reader 6.0 and above allow searching a collection of PDF files.

Using a search program to search for a text in a collection of files of different types, it may or may not be possible to also search PDF files, depending on the program. This is because the text is stored in coded form, and a program searching for some text must interpret the code and search the result, not just search the code. Search programs that do not work include that of Windows XP and Agent Ransack. However, for searching the Web, some search engines, such as Google and Yahoo!, include PDF files in searches. The option to view the PDF in HTML format is also commonly offered (this conversion does not include images).

Mac OS X, having PDF as a core element of the operating system, fully supports searching PDF files with the Preview application, used to view PDF files. The Spotlight feature in Mac OS X v10.4 extends this ability across the whole operating system, allowing information in PDF files (as well as almost all others) to be found from a single search box.

Types of content

A PDF file for e.g. a map is often a combination of vector graphics layer, text, and raster graphics, e.g., the general reference map of the US [7] uses:

An example of a PDF map without raster graphics is the CIA World Factbook's map of the Arctic. In the same publication's European map, the blue of the sea is not "filled" neatly to the vector graphics coast line, but just raster graphics, giving a cruder result (noticeable when highly zoomed in).

Tools exist, such as pdfimages (bundled with Xpdf) to extract the raster images from a PDF file. This can be extremely useful if the PDF is simply a collection of scanned pages.

See also

References

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.

External links

Tools

Commercial PDF Software

Free and open source software

Creator
Viewer
Editor

Closed source software

See also: Portable Document Format, 1990s, 2005, 2D computer graphics, Adobe Acrobat, Adobe InDesign, Adobe Systems, Agent Ransack