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Monsters from the wild
Some PDF writers produce PDFs which are not correct according to the specification.
Missing object
An attribute of an object has a reference pointing to a free reference in the cross references, i.e. the object is no in the file.
Treatment when writing out such object: an null as indirect object. This preserves the correct reference which may be used in several places. This will result in a type mismatch.
Seen in Producer
Microsoft® Excel® for Microsoft 365Bluebeam PDF Library 18
Incorrect stream length
The dictionary part of a stream has a Length which is different from the number of bytes between the stream and endstream tokens.
Seen in Producer
Bluebeam PDF Library 18