If the PDF has been signed by a digitial certificate or certified, the PDF has been locked to prevent changes or allow the detection of changes. Signnatures for PDF's were developed for commercial and legal documents and are covered by laws of many countries.
You need to have the signer to try to remove the signature or certificaton, work on an unsigned copy, or edit the source document.
Going forward, train and warn users that if they want to sign, password protectd, redact a document or other action that results in an unreversable action to keep a before copy that can be destroyed ata later date.