A File Identifier is described in section 8.3 of the PDF specification.
The first string is a permanent identifier based on the contents of the file
at the time it was originally created, and does not change as the file is
incrementally updated. The second string is a changing identifier based
on the file's contents the last time it was updated.
If this class were being use to update a PDF's file identifier, we'd need
to add a method to parse an existing file identifier.
Returns the CreatedPart as a byte array.
Returns the ModifiedPart as a byte array.