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    SECTION 2.3                                                             Creating PDF



2.3 Creating PDF

    PDF files may be produced either directly by application programs or indirectly
    by conversion from other file formats or imaging models. As PDF documents and
    applications that process them become more prevalent, new ways of creating and
    using PDF will be invented.

    Many applications can generate PDF files directly, and some can import them as
    well. This direct approach is preferable, since it gives the application access to the
    full capabilities of PDF, including the imaging model and the interactive and doc-
    ument interchange features. Alternatively, applications that do not generate PDF
    directly can produce PDF output indirectly. There are two principal indirect
    methods:

    • The application describes its printable output by making calls to an application
      programming interface (API) such as GDI in Microsoft® Windows® or Quick-
      Draw in the Apple Mac OS. A software component called a printer driver inter-
      cepts these calls and interprets them to generate output in PDF form.
    • The application produces printable output directly in some other file format,
      such as PostScript, PCL, HPGL, or DVI, which is converted to PDF by a sepa-
      rate translation program.

    Although these indirect strategies are often the easiest way to obtain PDF output
    from an existing application, the resulting PDF files may not make the best use of
    the high-level Adobe imaging model. This is because the information embodied
    in the application’s API calls or in the intermediate output file often describes the
    desired results at too low a level. Any higher-level information maintained by the
    original application has been lost and is not available to the printer driver or
    translator.

    Figures 2.1 and 2.2 show how Acrobat products support these indirect
    approaches. The Adobe PDF printer (Figure 2.1), available on the Windows and
    Mac OS platforms, acts as a printer driver, intercepting graphics and text opera-
    tions generated by a running application program through the operating system’s
    API. Instead of converting these operations into printer commands and transmit-
    ting them directly to a printer, the Adobe PDF printer converts them to equiva-
    lent PDF operators and embeds them in a PDF file. The result is a platform-
    independent file that can be viewed and printed by a PDF viewer application,
    such as Acrobat, running on any supported platform—even a different platform
    from the one on which the file was originally generated.

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