how to Print onto an existing PDF file?

I am printing medical test requests onto a preprinted request form from the pathology company

the preprinted form has all the company details and such on a A4 physical paper

I insert the paper into a printer, then I print the patient details and requests in the appropriate areas

I can scan the preprinted form into a PDF file on my computer

 How to I print on this scanned PDF file instead of the physical paper?

Currently, I print on the physical paper then have to scan the printed form to email the request

 thought I could save some Trees....