Document examination expert witness Ronald N. Morris is a certified forensic document examiner and in this excerpt from Submitting a Handwriting Case for Examination, he writes on working with copies:
Another reason why photocopies are problematic is the ability of a person to create or fabricate a completely fictitious document using photocopy technology. The purpose of fabricated documents is to offer them as proof of a position in a dispute, to justify a position, or taking some action. Increasingly, the authenticity of photocopied documents is being questioned because of the ease with which fictitious documents can be created to make it appear that the original document contained certain text or even a signature.
Assuming that the copy accurately records all of the features found on the original it purports to represent is hazardous at best. The only way to substantiate that a copy is an accurate reproduction of the document it purports to represent is by comparing the copy with the original. Authentication of an original based only on the examination of a purported copy of that document is not possible. Any opinion based on the examination of a copy applies only to the examined copy and cannot be extrapolated to the original the copy purports to represent.