Occasionally, you or the signer may choose to lock the document after signing. In this case, the document becomes read-only and no further changes are allowed; even a signature can’t be added in the document. The Lock document after signing option is useful if you’re the last signer or the only one signer.
For more information please check out the help page https://helpx.adobe.com/acrobat/kb/edit-signed-PDF.html#WhatifaPDFislockedaftersigning
Regards
Amal