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:
The best evidence for examination purposes is always the original document, but frequently only a photocopy is available. If it is necessary to examine a photocopy, the best copy for examination purposes is one made from the original document and not a copy of a copy.
Photocopies typically do not reveal all the evidence found on the original document or document being copied, i.e., significant quality and features of the writing, indentations, outlines, feather strokes, pen stops, alterations, etc. A photocopy can also contain artifacts not on the original. These artifacts may be dirt, dried white-out, or scratches on the glass. There may also be defects on the machine’s drum, or some other cause.
Fax documents, or copies of fax documents, are more problematic and many times are of little to no value for examination. Faxes or copies of faxes should only be submitted for examination purposes as a last resort. The value that any copy has for evaluation purposes is dependent on the quality of the copy, regardless of whether it is a photocopy or fax.