DER is still easy, UPER (unaligned packed encoding rules) is so much harder, yet it's prevalent in Telecom industry.
Last I checked, there was no freely available tool than can handle UPER l00%
Unfortunately, the protocols themselves can be confusing, badly (or worse: inconsistently) documented, and the binary formats often lack sensible backwards compatibility (or, even worse: optional backwards compatibility). Definitions are spread across different protocols (and versions thereof) and vendors within the space like to make their own custom protocols that are like the official standardised protocols, but slightly different in infuriating ways.
If you parser works (something open source rarely cares about so good luck finding one for your platform), the definitions extracted from those DOCX files are probably the least of your challenges.
For example, if you have ASN.1 UTF-8 string that is constrained to 52 specific characters - UPER encoding can present every character with 6 bits (not bytes).
In modern world you can apply zlib on top of UPER encoding or internal payload, however, depending on the use case.