A PDF Signature can embed a signature reference which specifies things about what changes to the document are allowed without invalidating the signature. It is

A PDF Signature can embed a signature reference which specifies things about what changes to the document are allowed without invalidating the signature. It is possible to make a signature that doesn't allow any changes to the file, or allows changes to annotations, or doesn't allow signature fields to be modified. To learn more google about "DocMDP" and "FieldMDP"