[ddulaney]

https://kaitai.io/ maybe?

It looks perfectly nice for its role, but I didn’t use it for my last project because I need serialization as well.

[elliottcarlson]

Kaitai is very useful for reverse engineering a binary file that you have some assumptions of. I've used it for save file reverse engineering and then creating a read/write library for it. It should be usable for PDF Metadata.