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SECTION 8.6                                                         Interactive Forms



FDF is based on PDF; it uses the same syntax (see Section 3.1, “Lexical Conven-
tions”) and basic object types (Section 3.2, “Objects”), and has essentially the
same file structure (Section 3.4, “File Structure”). However, it differs from PDF in
the following ways:

• The cross-reference table (Section 3.4.3, “Cross-Reference Table”) is optional.
• FDF files cannot be updated (see Section 3.4.5, “Incremental Updates”). Ob-
  jects can only be of generation 0, and no two objects can have the same object
  number.
• The document structure is much simpler than PDF, since the body of an FDF
  document consists of only one required object.
• The length of a stream may not be specified by an indirect object.
FDF uses the MIME content type application / vnd . fdf. On the Windows and
UNIX platforms, FDF files have the extension . fdf; on Mac OS, they have file type
' FDF '.


FDF File Structure

An FDF file is structured in essentially the same way as a PDF file but contains
only those elements required for the export and import of interactive form and
annotation data. It consists of three required elements and one optional element
(see Figure 8.10):

• A one-line header identifying the version number of the PDF specification to
  which the file conforms
• A body containing the objects that make up the content of the file
• An optional cross-reference table containing information about the objects in
  the file
• A trailer giving the location of various objects within the body of the file

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