I think it means that what should happen if the secured PDF is modified so as to alter the text (as opposed to adding annotation or a signature) the reading PDF reader should detect that the PDF body no longer passes it's hash check.
I presume the fixes are in either
a) The PDF generator, so the the hashed portion is separate from the signatures and annotation so that tampering is evident, or
b) in the reader so that it detects tampering correctly.
One hopes that it's not
c) Fix the PDF modifying app to not allow it to change the body text, honest gov I didn't touch it!