How can I remove "protection" from a document? PDF existing security settings override.

[Bryce Nesbitt]

I have a document that I own, downloaded from a government energy site, for which Properties->Security reads

Security: No Security
Printing: Allowed
Document assembly: Not Allowed
Change document: Not Allowed

I must submit this to a second government agency, who has complained they can't stamp it (e.g. "change it").
How can I strip the protections, short of printing to PDF?

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Similarly, if I try to add a password I get:

"This document contains digital signatures".  But how do I remove those digital signatures?