A user password is an open password
Until someone types it, the pages stay closed. That is different from owner restrictions (print/copy) — see Unlock if you have a file that opens but will not print. Here you are adding the open lock. Strength is the passphrase plus the PDF crypto pdf-lib applies.
Backup first
Duplicate the original. If you lock the only copy and forget the phrase, the content is gone. We cannot help.
Where the password lives
Only in your head, password manager, and the recipient’s out-of-band channel. We never see it. It is not stored after the Worker finishes.
Kids and shared tablets
A school PDF on a family iPad is a classic case: lock it so only a parent can open the file.