This property determines whether the file identifiers should be
constant rather than unique.
PDF documents have two unique file identifiers. One is a
file-specific identifier which stays constant throughout the life
of the file. The other is a file version identifier which is
updated every time the document is saved.
This property allows you to ensure that the same identifiers are
always used. This can be useful for debugging if you need to do a
binary-level comparison of files and you need to ensure that random
elements have been eliminated.