Well, the QRCode natively just has a bitmap form. The little server app requests that form (perhaps with a border, mirrored, reversed, etc.) and then magnifies it and converts it to a PNG. So, putting the result in a PDF is as trivial or as hard as putting a bitmap form into a PDF. How difficult that is is something that Olivier can perhaps answer.
QR codes ultimately just carry some text. We're used to it being a URL but any arbitrary text is possible. Several formats have established themselves. For instance, you can use the VCard format to create a business card QR code. So, there could be some nice uses of QR code where PDF is the right medium.
Cheers,
Jeff