The default platform being used for the file specification.
For historic reasons PDF has supported multiple different files
for multiple different platforms. This was appropriate when files
were often non-interoperable and would not port easily between
Fortunately this state of affairs is now unusual and only the
default "UF" platform is commonly used. Adobe do not specify what
the initials stand for, but it is likely to be Unicode File, though
it could be Universal File or URL File.
While you are unlikely to wish to create documents which use
these obsolescent platforms, you may find old documents that use
them. This method allows you to access information specified by
platform. Appropriate selectors are "UF", "F", "DOS", "Mac" or
"Unix" as per the descriptions in the Adobe PDF Specification. You
can use the Platforms property to deterine which are being
The defaut of "UF" provides access via the standard settings as
specified in the PDF 1.7 specification.