There was a bug in versions of Acrobat Reader from about 2018 (example version 2018.011.20055) where certain PDF documents, when modified and saved, would be given password protection and there was no way to know what the password was, so the document could never be opened again.
There is an update for Acrobat which fixes this problem. In Acrobat Reader use Help > Check for Updates.
Similar Issue in Preview
Note: there is a similar issue with Preview version 11. Fix with: sed -i '' -e 's/\/EncryptMetadata false/\/EncryptMetadata true/g' file.pdf
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