S E C T I O N H .1
1097
PDF Version Numbers
the viewer does not understand. (This describes the behavior in Acrobat 5.0 and
later; earlier versions do not notify the user.) If the viewer encounters an error, it
notifies the user that the document version is newer than expected, an error has
occurred, and no further errors will be reported. Acrobat permits a document
with a newer minor version to be inserted into another document.
Whether and how the version of a document changes when the document is
modified and saved depends on several factors. If the document has a newer ver-
sion than expected, the viewer does not alter the version—that is, a document’s
version is never changed to an older version. If the document has an older version
than expected, the viewer updates the document’s version to match the viewer’s
version. If a user modifies a document by inserting another document into it, the
saved document’s version is the most recent of the viewer’s version, the docu-
ment’s original version, and the inserted document’s version.
If the version of a document changes, viewers that are not PDF 1.4–aware cannot
save the document by using an incremental update because updating the header
requires rewriting the entire file. Among other disadvantages, rewriting the file
can cause existing digital signatures to become invalid. Since viewers that are
PDF 1.4–aware can use the
Version
entry in the document catalog to update the
document’s version, they can incrementally save the document (and will do so if
necessary to preserve existing signatures). For example, if an Acrobat 5.0 user
modifies a document that has a PDF version earlier than 1.4, the document can
be updated incrementally when saved (with the updated version of 1.4 in the doc-
ument catalog). However, if an Acrobat 4.0 user modifies a document that has a
PDF version earlier than 1.3, the entire file is rewritten when saved (with a new
header indicating version 1.3).
Again, the preceding discussion of viewer behavior applies to what the viewer
perceives to be a document’s PDF version, which may be different from the doc-
ument’s actual version if the viewer does not look for the
Version
entry in the
document’s catalog (a PDF 1.4 feature). One consequence is that a file may be re-
written when it could have been incrementally updated. For example, suppose an
Acrobat 4.0 user opens a document that has a version of 1.4 (newer than expect-
ed) specified in the catalog’s
Version
entry. Acrobat 4.0 determines the version by
looking only at the document’s header. There are two cases to consider:
•
The header specifies version 1.2 or earlier. If the user alters and saves the docu-
ment, the viewer updates the document’s version to match its own by rewriting
the file with a new header indicating version 1.3.