Tagged versus structured documents

If you consider content to be the leaves of a tree, the structure would be twigs and tags would be the branches and trunks. In the case of structured and tagged documents in Adobe® Acrobat® 6.0, all contain content, but there are additional components.

What’s in a document?

Look at a printed document. Notice the content, whether it's text or tables or images. Take note of the pagination elements, such as the page numbers and header and footer elements. Examine the layout elements, such as the figures.

Imagine that you could see the manuscript as we were editing it. You would see editing functions, such as text and graphic markups and comments. Now imagine that you could see the pages laid out for printing. What else do you see? Crop marks, print notations, and the like.

This is a basic document. This is also a description of an unstructured document. Not that there isn't a lot of visual structure, because there certainly is. However, this structure doesn't have to be programmed, nor can its source be reused automatically. Sure, you could copy and paste to your heart's content, but it's still unstructured.

The key to repurposing content lies in structure. In a PDF document, there are two levels of structured document: structured and tagged. Look at it as a continuum. At one end is a simple text document, at the other end is a tagged document, and somewhere in between lies a structured document.

Structured documents

Structured documents contain a structure tree in addition to the content. Content is referenced in a logical reading order, just as the pages of a book are structured. For example, this book uses a structured layout in that different levels of headings are used. So, when you read this paragraph, which has a lower-level heading than this subsection, you know that it's part of the larger heading, which in turn is part of the main headings. The screen shot below shows a portion of this chapter's preliminary outline view. You can easily see how the levels branch out from the main level—that is, the chapter title.

A structured document's layout uses a hierarchy to arrange elements.

A similar structure is in place from Acrobat's perspective. Some of the elements contained in a regular, unstructured document aren't used. For example, comments or page numbers aren't translated into a structured format because they aren't considered useful structures in terms of formulating outputs.

A basic structured document is one that contains elements such as bookmarks. If the document you're writing were converted to a PDF and opened in Acrobat, the bookmarked structure would look like the outline structure. As you can see above, we did convert the document, and the same structure is outlined in Acrobat.

A structured document in Acrobat uses bookmarks corresponding to the outline levels initially set in Word.

Tagged PDF documents

The most advanced form of document we look at in this chapter is the tagged document. Other elements are defined in addition to the content and structure already described. These other elements also relate to content, but from a different perspective. Content, as described earlier, relates to the words, tables, images, and so forth. In a tagged document, additional content information includes word spacing, hyphenation, and Unicode character values.

As you can see below, although the Tags Root heading is present and the headings displayed in the earlier two images appear, the tags don't have anything in the document assigned to them. On the other hand, if you had chosen to have the bookmarks tagged, the document's elements would appear here.

The logical structure tree in the Tags panel defines objects in a hierarchy.

Tagging has become critical in the world of repurposing documents. If a document isn't tagged, it will not currently reflow for the smaller screens of personal digital assistants or for use with screenreaders for the visually impaired.

As we've described, depending on the characteristics of your document, you can repurpose the content in different ways. The section below lists the three document types and summarizes the features each can support.

Repurposing capabilities of different PDF document types

Unstructured documents

• Save to RTF

• Paragraph recognition

Structured documents

• Save to RTF

• Paragraph recognition

• Text formatting

• View on screenreader (unreliable)

Tagged documents

• Save to RTF

• Paragraph recognition

• Text formatting

• Lists/tables

• Reflow ability

• View on screenreader (reliable)

Don't use an elephant gun to hunt a fly

I'll bet that got your attention. Sometimes, depending on the source materials you have available, it can take a good deal of time to plan and produce a tagged file and finesse components such as line breaks. But what if you don't need the tags? Don't use them. Again, the situation boils down to what you need.

Note: Tagging is an important method of structuring a document for accessibility. A tagged document can move straight from the paper world to special uses in the digital world.

Excerpted from "Adobe Acrobat 6: The Professional User's Guide." Copyright ©2003 by Donna L. Baker and Tom Carson, published by Apress. Used with the permission of Apress. To buy this book, visit www.amazon.com.