There are plenty of examples of a single set of documents with no corroborating documents. In fact, when I was a lawyer for Lehman in 2008 most mortgage transfers were a line in a spreadsheet not a legal document. Many wills have only a single copy that was kept in a safe deposit box. I could go on and on.

The fact remains that altering a photocopied or faxed copy of a document has been trivially easy for decades and yet it's not really a problem in real life, because it's a criminal act to do so and present it as authentic.