Hello Mairead,
The upcoming update should allow you do that. I will allow you have a unique QR code for each user who bought your document on the bottom right corner of each page.
if you purchase now the software will cost you 35 gbp, the upcoming week it will be available to old customers for free, for new customers at around 50.
It will allow deposits, file previews, shopping cart, qr codes and custom css3 buttons.
Thank you!