20200309 Documents can be encrypted with a user (aka open) password for privacy and security. A user password encrypted pdf document can be decrypted to an unencrypted pdf using:
$ pdftk d1.pdf input_pw <password> output d1_decrypted.pdf
$ qpdf --password=<password> -decrypt d1.pdf d1_decrypted.pdf
Documents might also have an owner (aka master or permissions) password.
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