Joomla extension modification: Digitoll Downloads for VM2

Howdy Developers! In my Joomla-VM2 shop I have both physical and virtual products as well. The virtual products are the physical products' voucher versions. So customers can download a PDF that represents the physical product. I attached these vouchers to some physical products and customers can select to buy the voucher version if they want. I managed this with the Digitoll Downloads extension: [login to view URL] I installed a specific plugin: "Download Codes for Digitoll Downloads - Text file" as well: [login to view URL] This gives me the possibility to attach unique codes to the downloads, so I can identify that the REAL buyer wants to redeem the voucher. This solution inserts the unique code inside the confirmation mail. It is working good so far. Here comes my problem: Since these are gift vouchers my clients would like to get the PDF vouchers with the download codes PRINTED ON the pdf file. So when they give the gift voucher to the celebrated, he/she can redeem the voucher that contains the download code. So the job is: you have to modify this extension, so it inserts the download code not inside the confirmation email, but in the downloaded PDF file somehow. Please ask me if something is not clear...