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      CHAPTER 3                                                                         Syntax



      The rules described here are sufficient to produce a well-formed PDF file.
      However, additional rules apply to organizing a PDF file to enable efficient
      incremental access to a document’s components in a network environment. This
      form of organization, called Linearized PDF, is described in Appendix F.

3.4.1 File Header

      The first line of a PDF file is a header identifying the version of the PDF
      specification to which the file conforms. For a file conforming to PDF 1.7, the
      header should be

         %PDF−1. 7

      However, since any file conforming to an earlier version of PDF also conforms to
      version 1.7, an application that processes PDF 1.7 can also accept files with any of
      the following headers:

         %PDF−1. 0
         %PDF−1. 1
         %PDF−1. 2
         %PDF−1. 3
         %PDF−1. 4
         %PDF−1. 5
         %PDF−1. 6

      (See also implementation notes 13 and 14 in Appendix H.)

      Beginning with PDF 1.4, the version in the file header can be overridden by the
      Version entry in the document’s catalog dictionary (located by means of the Root
      entry in the file’s trailer, as described in Section 3.4.4, “File Trailer”). This enables
      a PDF producer application to update the version using an incremental update
      (see Section 3.4.5, “Incremental Updates”).

      Under some conditions, a consumer application may be able to process PDF files
      conforming to a later version than it was designed to accept. New PDF features
      are often introduced in such a way that they can safely be ignored by a consumer
      that does not understand them (see Section H.1, “PDF Version Numbers”).

      Note: If a PDF file contains binary data, as most do (see Section 3.1, “Lexical Con-
      ventions”), it is recommended that the header line be immediately followed by a
      comment line containing at least four binary characters—that is, characters whose

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