“Competent scanners make a flat copy, then saving it in the document PDF preservation software that should also be flat.”
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Just as their replication apparently did. But that is structurally different that the forgery.
No clipping mask in the ‘replication’.
A scanner that applies a clipping mask has a name - defective.
If you open the “replication” on a Mac in Preview and select Save as PDF there will be a clipping mask.