This is not a fully qualified bug report, because I lack a reproducible example for a number of reasons. However, I originally posted in issue 416 about how I was experiencing decryption issue with a file generated by the Acrobat Sign product. It is a v1.7 PDF, with AES 128-bit encryption. That issue notes a merged fix (#1015).

I downloaded PyPDF2 2.10.0, which then gave a new error about missing PyCryptoDome, which I then installed (v3.15.0). Running my test case again, I received the following error:

ValueError: Data must be padded to 16 byte boundary in CBC mode

I know little about PDF specs, and even less about encryption, however using the PyCryptoDome docs I did find that the following code addition to _encryption.py alleviated this issue for my file:

86a87,89
>             if len(data) % 16:
>                 from Crypto.Util.Padding import pad
>                 data = pad(data, 16)

Again, lacking a reproducible test case I'm not sure how useful this is but wanted to share my findings in case someone with access to Acrobat Sign can generate a file that demonstrates the same behavior.