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                                                  107
       SECTION 3.4                                                                   File Structure



       files that are not hybrid-reference files; see “Compatibility with Applications That
       Do Not Support PDF 1.5” on page 109), the keywords xref and trailer are no longer
       used. Therefore, with the exception of the startxref address %%EOF segment and
       comments, a PDF 1.5 file is entirely a sequence of objects.

       Note: The use of object streams and cross-reference streams is permitted in linear-
       ized PDF, with minor modifications to the specification (see Section F.2, “Linearized
       PDF Document Structure”).


       Cross-Reference Stream Dictionary

       Cross-reference streams contain the entries shown in Table 3.15 in addition to
       the entries common to all streams (Table 3.4) and trailer dictionaries (Table 3.13).
       Since some of the information in the cross-reference stream is needed by the
       consumer application to construct the index that allows indirect references to be
       resolved, the entries in cross-reference streams are subject to the following
       restrictions:

       • The value of all entries shown in Table 3.15 must be direct objects; indirect ref-
         erences are not permitted. For arrays (the Index and W entries), all their ele-
         ments must be direct objects as well. If the stream is encoded, the Filter and
         DecodeParms entries in Table 3.4 must also be direct objects. Also, see imple-
         mentation note 20 in Appendix H.
         Note: Other cross-reference stream entries not listed in Table 3.15 may be indi-
         rect; in fact, some (such as Root in Table 3.13) are required to be indirect.
       • The cross-reference stream must not be encrypted, nor may any strings appear-
         ing in the cross-reference stream dictionary. It must not have a Filter entry that
         specifies a Crypt filter (see 3.3.9, “Crypt Filter”).

            TABLE 3.15 Additional entries specific to a cross-reference stream dictionary
KEY        TYPE          DESCRIPTION

Type       name          (Required) The type of PDF object that this dictionary describes; must be XRef for
                         a cross-reference stream.

Size       integer       (Required) The number one greater than the highest object number used in this
                         section or in any section for which this is an update. It is equivalent to the Size en-
                         try in a trailer dictionary.

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