Trouble digitally signing

I have a pdffiller document that is saved as a PDF. There is a signature line. When I click on the signature line (to sign the document) it gives me the option to choose the digital ID that I want to use for signing. I click that and hit continue. Then I click the sign button. That brings up the save as box. I change the name of the document, save as type, adobe, PDF files, then click on save. Once I do that an error information box comes up. It states "creation of the signature could not be completed. Internal general error." Then I click OK , then adobe acrobat sends a message "the save operation was canceled". So, I hit OK then adobe acrobat box comes up "the document could not be saved" and I hit OK. Why can't I sign this document?