SECTION 3.4
93
File Structure
codes are 128 or greater. This ensures proper behavior of file transfer applications
that inspect data near the beginning of a file to determine whether to treat the file’s
contents as text or as binary.
3.4.2 File Body
The
body
of a PDF file consists of a sequence of indirect objects representing the
contents of a document. The objects, which are of the basic types described in
pages, and sampled images. Beginning with PDF 1.5, the body can also contain
object streams, each of which contains a sequence of indirect objects; see Section
3.4.3 Cross-Reference Table
The
cross-reference table
contains information that permits random access to
indirect objects within the file so that the entire file need not be read to locate any
particular object. The table contains a one-line entry for each indirect object,
specifying the location of that object within the body of the file. (Beginning with
PDF 1.5, some or all of the cross-reference information may alternatively be
contained in cross-reference streams; see Section 3.4.7, “Cross-Reference
The cross-reference table is the only part of a PDF file with a fixed format, which
permits entries in the table to be accessed randomly. The table comprises one or
more
cross-reference sections.
Initially, the entire table consists of a single section
(or two sections if the file is linearized; see Appendix F). One additional section is
added each time the file is updated (see Section 3.4.5, “Incremental Updates”).
Each cross-reference section begins with a line containing the keyword
xref
.
Following this line are one or more
cross-reference subsections,
which may appear
in any order. The subsection structure is useful for incremental updates, since it
allows a new cross-reference section to be added to the PDF file, containing
entries only for objects that have been added or deleted. For a file that has never
been updated, the cross-reference section contains only one subsection, whose
object numbering begins at 0.