The Signature of the customer is stored in the agreement file as a PDF. To open the file, find the donor in Donor Manager and click the blue tag under AGREEMENT in the data pane.
Contents of the PDF file can be rewritten by using 'Rewrite PDF' command in 'Add Event' box. This is the file to be exported when 'PDF' is selected in the Secure Export module.
There is no need to rewrite this PDF file when you modify Start Date for the donor, because it will be rewritten automatically.