I am using Adobe Acrobat 9 Pro Extended, client uses Adobe 7. Client sends a digitally signed pdf document , I in turn digitally sign and return to them. When they open the document they only see a black box, yet on my end I see both digital signature.

I am using Adobe Acrobat 9 Pro Extended software on a PC, client uses adobe 7. Client sends a digitally signed document which in turn I sign and return. When they open the document they only get a black box instead of my digital signature. On my end I can see both digital signatures?