Portable Document Format Technology PostScript History

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 PS interpreter. However the entire PS 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 tokenizeTokenizing is the operation of splitting up a string of characters into a set of tokens. The term is also used when during the parsing of source code of some programming languages, the symbols are converted into another format that is much smaller. Most Bd, 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. 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.

1.2 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 ( AcrobatAdobe Acrobat was the first software to support Adobe Systems' Portable Document Format. It is mostly described in those entries. The Acrobat Reader program (now just called Adobe Reader) is available as a no-charge download from Adobe's web site, and all) cost money, but so did the software to view and print PDF files. Additionally, there were competing formats. 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.

1.3 Macintosh

PDF was selected as the "native" metafile format for Mac OS XMac OS X is the latest version of the Mac OS operating system for Macintosh computers. Developed and published by Apple Computer, it provides the stability of a Unix operating environment and adds popular features of the traditional Macintosh user interfa, replacing the PICTPICT is a graphics file format introduced on the original Apple Macintosh computer as its standard metafile format. It allows the interchange of graphics (both bitmapped and vector), and some limited text support, between Mac applications, and was the nat format of the earlier Mac OSMac OS which stands for Mac intosh O perating S ystem, is Apple Computer’s name for the operating systems for Macintosh computers. The original Mac OS was the first commercially successful operating system which used a graphical user interface. The Macint. Mac OS X's imaging model, Quartz 2DQuartz 2D is the primary graphics library in Mac OS X, it supersedes QuickDraw which was used in earlier versions of the Mac OS, now known as "Classic". Quartz 2D is based on version 1. 4 of the Adobe Portable Document Format (PDF — a universal file forma, is based on both the Display PostScript standard and PDF, and is sometimes referred to as DisplayPDF . Due to OS support, all OS X applications can create PDF documents automatically as long as they support the Print command.

2 PDF and accessibility

PDF is not intrinsically inaccessible 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. Moreover, that process itself is inaccessible. Nonetheless, well-made PDFs can be a valid choice as long-term accessible documents.

3 Comparison between PDF and HTML

PDF and HTML are not equivalent technologies, but are both commonly found on the Web.

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. PDF 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 has become 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:

Enlarging a PDF document magnifies the text but preserves the original layout and spacing; a practical limit on zooming follows from the requirement to keep a text column within the width of the screen (otherwise horizontal scrolling would be needed during and after reading each line, which would be very cumbersome).

With HTML, a larger font size is used and lines re-wrap accordingly to fit the browser window. Note, though, that tagged PDFs can be reflowed quite well, keeping columns of text within the viewable area.

4 Searching for a text in a collection of files

Adobe Acrobat Reader 6.0 allows 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, Google includes PDF files in searches. It offers the option "View as HTML" (this conversion does not include images).

Mac OS X, having PDF as a core part of the OS, fully supports searching PDF files with the Preview application, used to view PDF files. The Spotlight feature in the upcoming OS X 10.4 promises to extend 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.

5 Layers

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

vector graphics for coastlines, lakes, rivers, highways, markings of cities, and Interstate highway symbols – on zooming in, the curves remain sharp, they do not appear as consisting of enlarged pixels (i.e. rectangles of pixels)
text stored as such – scalable, and also one can copy the text
raster graphics for showing mountain relief – on zooming in, this consists of enlarged pixels (the blue of the sea and lakes is "filled" neatly to the vector graphics coast line, hence not part of this raster graphics layer)

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 part of the raster graphics layer, giving a cruder result (noticeable when highly zoomed in).

6 See also

7 References

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

8 External links

Adobe: Postscript vs. PDF (Dead link as of 2004)
- Adobe: Postscript vs. PDF ( Archive.org copy)
PDF Specification also available as a book describing PDF 1.4 (BooksEnthsiast.com)
PDF/X Frequently asked questions
Planet PDF
PDF zone
- The Camelot Project
PDF-Worker – A directory of PDF programs and resources.
Online viewer for PDF, PostScript and Word

8.1 Tools

Doc2pdf – Conversion by email robot.
Go2PDF – Freeware virtual printer driver.
GNOME PDF Viewer
iText – Converter in Java.
gnujpdf – Java converter.
PJX A Java package for general PDF programming.
libharu – Converter in C/C++.
OpenOffice.org – Can transform many types of documents into PDF documents.
OttoPDF
Panda library
PDFCreator – A GPL PDF printer driver for Windows.
PDF/X-3
sharpPDF – C# PDF library.
Xpdf – A PDF viewer.
PDFTOHTML – A PDF to HTML convertor. Based on Xpdf.
WikiPDF – A tool to convert MediaWiki articles to PDF.
POM - An open source java tool for merging PDF files.

Adobe Systems Computer file formats Graphics file formats

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Home > Portable Document Format

1 Technology

1.1 PostScript