PortableSigner is a signing (with X.509 certificates) program for PDF files. It's platform independent and runs (tested) under Windows (2000, XP, ...), Linux, and macOS.
It's possible to sign PDF documents digital with X.509 certificates. This signed documents are read only. Therefore it's possible to implement "electronic paper".
From the developers website:
"Last Update 2013..."
Use with Extreme Caution
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