Add a signature to a PDF file signed with Acrobat Sign

After signing with Acrobat Sign PDFs usually are locked and cannot be further manipulated without breaking the existing signature. 

So is the Acrobat Sign signing workflow already finished and you received the final result thereof? Then you can likely not add your signature anymore.

Or is the Acrobat Sign workflow still in progress and add part thereof it's now your turn to sign? Then follow the instructions you got with the notification that it's your turn.