Not able to digitally sign in Acrobat DC.

I received a signed PDF document requiring two authorizing signatures which does not allow modification (checked in File:Preferences).  I created a self signed digital ID with Acrobat DC and set the signature as the default.  When the document is opened in Sign & Fill the only option available is to validate the existing digital signature.  All other signing options are grayed out.  I have tried saving a copy of the file using File:Save As, but I could not see any option with that method control further modification.  How does one add another digital signature to the file?