Gets the set of IDs which have been processed.
During validation many IndirectObjects will be encountered.
Each time this happens, the ID value is added to this
The result is a set of IDs which allows you to determine the
Objects that were validated.
If some are not validated this will be generally because they
were not referenced anywhere.
However it may also be that they were referenced but that the
references were not part of the PDF specification and thus could
not be followed.