In Is it Fabricated?, document examination expert witness Ronald N. Morris writes on what material can be on a copy in addition to the material on the original.
First, copies may include toner reproduction of scratches, trash, dirt, etc., found on the glass or drum. Based on the examination and evaluation of the copy, original being copied, and the copier, it may be possible to determine the source of these extraneous marks and features on the copy. Second, photocopies are not always able to reproduce:
a. All of the qualities and features of the original writing, i.e, delicate and lightly written strokes found in normal, natural writing; and occasionally found in patching and retouching of individual strokes, letters, fine tremor movements, stroke direction, substituted letters, etc. Notwithstanding the improvement in copier design, image processing, and reproduction, copiers still do not exactly reproduce all of these qualities and features. If the copy is a first generation copy, the copy quality is typically better than that of a second, third or fourth generation copy.