About tags, accessibility, reading order, and reflow
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PDF
tags are similar in many ways to XML tags. PDF tags indicate document structure:
which text is a heading, which content makes up a section, which
text is a bookmark, and so on. A logical structure tree of tags
represents the organizational structure of the document. Thus tags
can indicate the precise reading order and improve navigation—particularly
for longer, more complex documents—without changing the appearance
of the PDF.
Assistive software determines how to present and interpret the
content of the document by using the logical structure tree. Most
assistive software depends on document structure tags to determine
the appropriate reading order of text and to convey the meaning
of images and other content in an alternate format, such as sound.
An untagged document does not have structure information, and Acrobat must infer a structure based
on the Reading Order preference setting. This situation often results
in page items being read in the wrong order or not at all.
Reflowing a document for viewing on the small screen of a mobile
device relies on these same document structure tags.
 Often, Acrobat tags PDFs when you create them.
To determine whether a PDF contains tags, choose File > Properties,
and look at the Tagged PDF value in the Advanced pane of the Description
tab.
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