Well you do need really expensive software for it, but considering there are free PDF readers, I imagine someone by now must have the format figured out, so they'd be able to write an open source editor that also does not leave traces. That would probably be super illegal due to IP laws though, which is why it has not been done.
Anything done can a computer can be edited without leaving any traces, it's just a matter of how hard it is to do.
For something that absolutely needs to be tamper proof nothing beats a physical media, like paper with some kind of tamper seal. Suppose even that is not going to be 100% secure though.
Seems to me this is not a document format thing but a protocol thing. The bank's process of accepting documents needs to be more secure if they are scared someone can edit it in transit.