Fix PDF test files

[andiwand]

[andiwand]

as a little bit expected object references can occur all over the place and should rather be handled by another abstraction like an object cache which will recurse as far as necessary and resolve all the references into objects

the cache would also save us from reading the same object multiple times from file

a references seems not to be a primitive type after all but I think it is still a useful abstraction in our current implementation