Bill:
Is this happening for all Customers or just this one? If just one, it could be that the attachment is being removed at their end.

Put Adagio aside for the moment. What happens if you go into Outlook, create a new email with a PDF attachment and send it to the Customer? If it goes through OK, create a new email again to the same customer but attach the PDF file from the email created by Adagio earlier and send it.