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    SECTION 2.4                                          PDF and the PostScript Language




                                        PostScript
                                     page description




                                     Acrobat Distiller


                                           PDF


                                         Acrobat




                    FIGURE 2.2 Creating PDF files using Acrobat Distiller


2.4 PDF and the PostScript Language

    The PDF operators for setting the graphics state and painting graphics objects are
    similar to the corresponding operators in the PostScript language. Unlike Post-
    Script, however, PDF is not a full-scale programming language; it trades reduced
    flexibility for improved efficiency and predictability. PDF therefore differs from
    PostScript in the following significant ways:

    • PDF enforces a strictly defined file structure that allows an application to ac-
      cess parts of a document in arbitrary order.
    • To simplify the processing of content streams, PDF does not include common
      programming language features such as procedures, variables, and control con-
      structs.
    • PDF files contain information such as font metrics to ensure viewing fidelity.
    • A PDF file may contain additional information that is not directly connected
      with the imaging model, such as hypertext links for interactive viewing and
      logical structure information for document interchange.

    Because of these differences, a PDF file generally cannot be transmitted directly
    to a PostScript output device for printing (although a few such devices do also

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