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                                                  63
        SECTION 3.2                                                                       Objects



KEY            TYPE                   VALUE

DL             integer                (Optional; PDF 1.5) A non-negative integer representing the number
                                      of bytes in the decoded (defiltered) stream. It can be used to deter-
                                      mine, for example, whether enough disk space is available to write a
                                      stream to a file.
                                      This value should be considered a hint only; for some stream filters, it
                                      may not be possible to determine this value precisely.


  3.2.8 Null Object

        The null object has a type and value that are unequal to those of any other object.
        There is only one object of type null, denoted by the keyword null. An indirect
        object reference (see Section 3.2.9, “Indirect Objects”) to a nonexistent object is
        treated the same as a null object. Specifying the null object as the value of a
        dictionary entry (Section 3.2.6, “Dictionary Objects”) is equivalent to omitting
        the entry entirely.


  3.2.9 Indirect Objects

        Any object in a PDF file may be labeled as an indirect object. This gives the object
        a unique object identifier by which other objects can refer to it (for example, as an
        element of an array or as the value of a dictionary entry). The object identifier
        consists of two parts:

        • A positive integer object number. Indirect objects are often numbered sequen-
          tially within a PDF file, but this is not required; object numbers may be
          assigned in any arbitrary order.
        • A non-negative integer generation number. In a newly created file, all indirect
          objects have generation numbers of 0. Nonzero generation numbers may be in-
          troduced when the file is later updated; see Sections 3.4.3, “Cross-Reference
          Table,” and 3.4.5, “Incremental Updates.”

        Together, the combination of an object number and a generation number
        uniquely identifies an indirect object. The object retains the same object number
        and generation number throughout its existence, even if its value is modified.

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