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                                   APPENDIX H


                  Compatibility and
                     H




                Implementation Notes
    The goal of the Acrobat family of products is to enable people to exchange and
    view electronic documents easily and reliably. Ideally, this means that any Acro-
    bat viewer application should be able to display the contents of any PDF file, even
    if the PDF file was created long before or long after the viewer application. Of
    course, new versions of viewer applications are introduced to provide additional
    capabilities not present before. Furthermore, beginning with Acrobat 2.0, viewer
    applications may accept plug-in extensions, making some Acrobat viewers more
    capable than others, depending on what extensions are present.

    Both viewer applications and PDF have been designed to enable users to view ev-
    erything in the document that the viewer application understands and to ignore
    or inform the user about objects not understood. The decision whether to ignore
    or inform the user is made on a feature-by-feature basis.

    The original PDF specification did not define how a viewer application should
    behave when it reads a file that does not conform to the specification. This ap-
    pendix provides that information. The PDF version associated with a file deter-
    mines how it should be treated when a viewer application encounters a problem.

    In addition, this appendix includes notes on the Acrobat implementation for de-
    tails that are not strictly defined by the PDF specifications.


H.1 PDF Version Numbers

    PDF version numbers take the form M . m, where M is the major and m the minor
    version number. Adobe increments the major version number when the PDF
    specification changes in such a way that existing viewer applications are unlikely
    to read a document without serious errors that prevent pages from being viewed.


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