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                                                       98
           CHAPTER 3                                                                              Syntax



KEY         TYPE           VALUE

ID          array          (Optional, but strongly recommended; PDF 1.1) An array of two byte-strings consti-
                           tuting a file identifier (see Section 10.3, “File Identifiers”) for the file. The two byte-
                           strings should be direct objects and should be unencrypted. Although this entry is
                           optional, its absence might prevent the file from functioning in some workflows
                           that depend on files being uniquely identified.

           Note: Table 3.17 defines an additional entry, XRefStm, that appears only in the trail-
           er of hybrid-reference files, described in “Compatibility with Applications That Do
           Not Support PDF 1.5” on page 109.

           Example 3.7 shows an example trailer for a file that has never been updated (as
           indicated by the absence of a Prev entry in the trailer dictionary).

           Example 3.7

              trailer
                  << /Size 22
                      /Root 2 0 R
                      /Info 1 0 R
                      /ID [ < 81b14aafa313db63dbd6f981e49f94f4 >
                            < 81b14aafa313db63dbd6f981e49f94f4 >
                          ]
                  >>
              startxref
              18799
              %%EOF


     3.4.5 Incremental Updates

           The contents of a PDF file can be updated incrementally without rewriting the
           entire file. Changes are appended to the end of the file, leaving its original
           contents intact. The main advantage to updating a file in this way (as discussed in
           Section 2.2.7, “Incremental Update”) is that small changes to a large document
           can be saved quickly. There are additional advantages:

           • In some cases, incremental updating is the only way to save changes to a docu-
             ment. An accepted practice for minimizing the risk of data loss when saving a
             document is to write it to a new file and rename the new file to replace the old
             one. However, in certain contexts, such as when editing a document across an
             HTTP connection or using OLE embedding (a Windows-specific technology),

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