I would advise you to deliver your incredible goodwill to a non-profit organization which actually values it instead of working on something for banks :)

The rotten, soulless entities that is banks do not deserve ANY free work.

They don't care about you, they won't value you work, they won't give anything back.

They'll use it to maximize their profits at best.

But most likely, they won't do anything: With absolute certainty, they KNOW that PDFs can be signed. They have to deal with cryptography anyway, and have security consulting.

They very likely intentionally decided to not sign PDFs anyway just because they can get away with it without getting sued, and save money by that.