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     CHAPTER 10                                                     Document Interchange



     same value. If both identifiers match when a file reference is resolved, it is very
     likely that the correct file has been found. If only the first identifier matches, a
     different version of the correct file has been found.

     To help ensure the uniqueness of file identifiers, it is recommend that they be
     computed by means of a message digest algorithm such as MD5 (described in In-
     ternet RFC 1321, The MD5 Message-Digest Algorithm; see the Bibliography), us-
     ing the following information (see implementation note 163 in Appendix H):

     • The current time
     • A string representation of the file’s location, usually a pathname
     • The size of the file in bytes
     • The values of all entries in the file’s document information dictionary (see
       Section 10.2.1, “Document Information Dictionary”)


10.4 Page-Piece Dictionaries

     A page-piece dictionary (PDF 1.3) can be used to hold private application data.
     The data can be associated with a page or form XObject by means of the optional
     PieceInfo entry in the page object (see Table 3.27 on page 145) or form dictionary
     (see Table 4.45 on page 358). Beginning with PDF 1.4, private data may also be
     associated with the PDF document by means of the PieceInfo entry in the docu-
     ment catalog (see Table 3.25 on page 139).

     Applications can use this dictionary as a place to store private data in connection
     with that document, page, or form. Such private data can convey information
     meaningful to the application that produces it (such as information on object
     grouping for a graphics editor or the layer information used by Adobe Photo-
     shop®) but is typically ignored by general-purpose PDF viewer applications.

     As Table 10.5 shows, a page-piece dictionary may contain any number of entries,
     each keyed by the name of a distinct application or of a well-known data type rec-
     ognized by a family of applications. The value associated with each key is an ap-
     plication data dictionary containing the private data to be used by the application.
     The Private entry may have a value of any data type, but typically it is a dictionary
     containing all of the private data needed by the application other than the actual
     content of the document, page, or form.

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