There is no problem with pulling up a previously saved background, and applying it to the PDF. The problem is that the background file needs to be specified each time (from the usual file-open dialogue).
There are printers on which you can specify a permanent background, but Adobe PDF is not one of those (as far as I can see).
Curiously, we never actually save the PDF. We get the PDF generated, at which point Adobe Reader pops up showing the PDF, and we click on the Email button in the Reader and enter the customer's address.
The whole purpose of this is to email an invoice as a PDF. We don't keep the PDF ourselves.
Typically this process is used for Proforma invoices. The real invoices always go in the mail...
We prefer to fax proformas but a lot of people are too lazy to walk to their fax machine and they insist on the invoice being emailed... this also saves paper.