If you are unable to lock a pdf file on Samsung phone then you can do this as --
1) download and install Samsung notes
2) provide it with required permissions
3)tap on pdf like icon
4) then add the file which you want to lock.
5) open the files in by using Samsung notes.
6) tap on the three dots option in top right part.
7) tap on lock like icon
set your password you can also set your file to be unlocked by fingerprint.
If it really helped then like and comment.
