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                                              30
      CHAPTER 1                                                              Introduction



      • The ability to provide table summaries associated with table structures. This
        feature can help a visually impaired person understand the purpose and struc-
        ture of a table without having to read the content in that table.
      • The ability to identify background page artifacts, which can be important to
        document reflowing. Background artifacts are collections of objects that do not
        contribute to the meaning of the author's original content, such as a colored
        rectangle behind a sidebar or a full-page background image. Such page back-
        grounds may not correlate to any logical structure, but they may be useful in
        reproducing the appearance of original document.
      • The ability to differentiate the pagination artifacts: watermarks, headers and
        footers.


1.2.4 Document Navigation Feature

      Additions to document navigation specify the viewing and organizational charac-
      teristics of portable collections, in which multiple file attachments are displayed
      within a single window. Portable collections are used to present, sort, and search
      collections of related documents, such as email archives, photo collections, and
      engineering bid sets.


1.2.5 Security-Related Features

      Additions to PDF introduced in 1.7 increase the control the document author can
      impose upon digital signatures and over requirements PDF consumer applica-
      tions must satisfy:
      • Additional digital signature constraints, which are enforced at the time the sig-
        nature is applied. These constraints include preferred digest methods, revoca-
        tion checking of the certificate used in a signature, and flags that clarify the
        interpretation of other parameters.
      • Additional constraints regarding the certificate to be used when signing. These
        constraints include Subject Distinguished Name (DN) dictionaries that must
        be present in the certificate, KeyUsage extensions that must be present in the
        signing certificate, and flags that clarify the interpretation of other parameters
        that specify certificate constraints.
      • The ability to specify requirement handlers that verify some requirement that
        the PDF consumer applications must satisfy before processing or displaying a
        PDF document. This feature provides an approach that ensures backward com-

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