Patterico points out that, in the space designated "Sign Here", it's stamped "Client Copy" in place of anyone's actual signature.
Which suggests that Trump himself, or someone with access to his personal papers, leaked it.
How do we know it's Trump's for-real 2005 form 1040?
It's illegal to file a false tax return with the IRS. But this isn't the file copy. It's not even signed. Not by Trump, and not by the paid preparer.
Is it somehow impossible, or even illegal, for Trump (or for that matter Betty Sue Derkins of Pumpkintown, SC) to fill out a 2005 form 1040 in Trump's name, stamp it "Client Copy", and anonymously send a photocopy to someone who is not any sort of government official?
So, really, we have no reason to believe anything on that piece of paper.