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      CHAPTER 2                                                                  Overview



        users; the users’ access can be selectively restricted to allow only certain opera-
        tions, such as viewing, printing, or editing.
      • The document can be digitally signed to certify its authenticity. The signature
        may take many forms, including a document digest that has been encrypted
        with a public/private key, a biometric signature such as a fingerprint, and oth-
        ers. Any subsequent changes to a signed PDF file invalidate the signature.


2.2.7 Incremental Update

      Applications may allow users to modify PDF documents. Users should not have
      to wait for the entire file—which can contain hundreds of pages or more—to be
      rewritten each time modifications to the document are saved. PDF allows modifi-
      cations to be appended to a file, leaving the original data intact. The addendum
      appended when a file is incrementally updated contains only those objects that
      were actually added or modified, and includes an update to the cross-reference
      table. Incremental update allows an application to save modifications to a PDF
      document in an amount of time proportional to the size of the modification rath-
      er than the size of the file.

      In addition, because the original contents of the document are still present in the
      file, it is possible to undo saved changes by deleting one or more addenda. The
      ability to recover the exact contents of an original document is critical when digi-
      tal signatures have been applied and subsequently need to be verified.


2.2.8 Extensibility

      PDF is designed to be extensible. Not only can new features be added, but appli-
      cations based on earlier versions of PDF can behave reasonably when they en-
      counter newer features that they do not understand. Appendix H describes how a
      PDF consumer application should behave in such cases.

      Additionally, PDF provides means for applications to store their own private in-
      formation in a PDF file. This information can be recovered when the file is im-
      ported by the same application, but it is ignored by other applications. Therefore,
      PDF can serve as an application’s native file format while its documents can be
      viewed and printed by other applications. Application-specific data can be stored
      either as marked content annotating the graphics objects in a PDF content stream
      or as entirely separate objects unconnected with the PDF content.

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