By Duff Johnson, Planet PDF Contributing Editor

PDF files can contain many things. At a minimum, a PDF contains the text, graphics, bookmarks, links and other elements of content that go to make up an electronic document.

In addition to content, PDF files may also include "structure". Structure is the term for a set of instructions that define the logic that binds the content together - the correct reading order, for example, and the presence and meaning of significant elements such as figures, lists, tables, and so on.

In PDF files, structure is expressed via "tags". Tags may be generated automatically for any PDF file using Acrobat 6.0 Professional, but unless the document is very simple indeed, automated tagging alone is unlikely to produce satisfactory results, and is certainly not a quick-fix for compliance with Section 508.

A PDF file equipped with well-formed tags may be "reflowed" to fit different page or screen widths, and will display well on handheld devices. Tagged PDF files also work better with the screen-reader devices used by many blind and other disabled users. In most cases, tags are necessary in order to make a PDF file comply with Section 508.

Adobe's Acrobat 6.0 will add tags to a PDF file, but human intelligence is still required to ensure the tagging process was performed correctly. There is little room for error in document tagging. Even seemingly small errors in document structure can easily render a file completely incomprehensible.

Imagine using your computer with the screen turned off, and you'll get some idea of how important logical text-flow is to anyone who needs a screen-reader to read your PDF!

Characteristics of a properly tagged PDF:

• The PDF file includes a logical reading order for its content
• Images are given correct alternate descriptions
• Tables are correctly tagged to represent the table structure
• Form-fields are authored to promote their utility to screen-readers
• Represents text as Unicode to clear up composition irregularities such as soft and hard hyphens