Leo says that what Clinton needs is a custom form application that will allow him to update the work order and then resend it via PDF. There is a program for iPad called PDF Pen, and he can edit it and sign it on there.
Since he wants to create an invoice with that, he might want to look into FileMaker Go. This is a database program that would let him develop custom databases and forms. Then he can output it and email it to a computer with Filemaker Pro to do the rest.