The first question to consider when assessing whether something is a forgery is, what's to be gained by the forger? The gain of counterfeiting money is obvious, and a newly discovered will benefits someone, but I'm trying to imagine the motivation for creating a bunch of letters written by a guy who died a century before.
Was there a prayer meeting where someone declared, "That's not what Paul said!" and someone shows up at the next meeting with a well worn papyrus scroll and says, "Oh yeah. Read it for yourself."
It's certainly plausible that the second century Christians only had copies of copies of Paul's letters, with all the errors inherent in that process, but that's not forgery, however they might diverge from the original.