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       CHAPTER 10                                                    Document Interchange



       tion, either through a screen reader or by some more direct invocation of a text-
       to-speech engine, PDF supports the following features:

       • Specifying the natural language used for text in a PDF document—for example,
         as English or Spanish, or used to hide or reveal optional content (see Section
         10.8.1, “Natural Language Specification”)
       • Providing textual descriptions for images or other items that do not translate
         naturally into text (Section 10.8.2, “Alternate Descriptions”), or replacement
         text for content that does translate into text but is represented in a nonstandard
         way (such as with a ligature or illuminated character; see Section 10.8.3,
         “Replacement Text”)
       • Specifying the expansion of abbreviations or acronyms (Section 10.8.4, “Ex-
         pansion of Abbreviations and Acronyms”)

       The core of this support lies in the ability to determine the logical order of con-
       tent in a PDF document, independently of the content’s appearance or layout,
       through logical structure and Tagged PDF, as described under “Page Content
       Order” on page 889. An accessibility application can extract the content of a doc-
       ument for presentation to users with disabilities by traversing the structure hier-
       archy and presenting the contents of each node. For this reason, producers of
       PDF files must ensure that all information in a document is reachable by means
       of the structure hierarchy, and they are strongly encouraged to use the facilities
       described in this section.

       Note: Text can be extracted from Tagged PDF documents and examined or reused
       for purposes other than accessibility; see Section 10.7, “Tagged PDF.”

       Additional guidelines for accessibility support of content published on the Web
       can be found in the W3C document Web Content Accessibility Guidelines and the
       documents it points to (see the Bibliography).


10.8.1 Natural Language Specification

       Natural language can be specified for text in a document or for optional content.

       The natural language used for text in a document is determined in a hierarchical
       fashion, based on whether an optional Lang entry (PDF 1.4) is present in any of
       several possible locations. At the highest level, the document’s default language
       (which applies to both text strings and text within content streams) can be speci-

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