Then your explanation belies the excuse the reporter gave in “finding it in his mailbox.”
Which one is it? They cannot play both sides of the street on this. Seems to me that the reporter already gave the provenance excuse away and with it any excuse that anyone “vetted it” in any manner.
Nothing would prevent someone from using any ordinary desktop publishing software to fabricate a tax return and fill in fictitious numbers. In fact, I wouldn't be shocked if these media outlets receive dozens of such things every month ... 1099 forms for Donald Trump from "Vladimir Putin," invoices for construction services at a Trump property in the Middle East from "Osama bin Laden," etc.
The reporter had to have some way of verifying the accuracy of the information he received.
P.S. -- Why would you assume that the reporter was telling the truth about how he got the document, anyway?