1. customer makes an order - with no confirmation from shop, yet. So, customer will not get invoice, but "offer" - in pdf. It is a regular order - with vendor, ordered items, addresses billto/ shipto. etc. With similar format to invoice.
2. When particular "offer" is accepted, confirmation to be done. Only now the invoice is created.
At step 1. i cannot generate any PDF file - due to fact that invoice number is not set. (step 2) - AFAIK.
So, from this point comes an idea of "forking" invoice view into separate "offer" view.