I have a poster i put up on gaming conventions. On that poster I have QR codes for downloading issues of my fanzine. Links from QR codes lead directly to pdf, mobi and epub files so people can download the zine and read it on the spot.
Piwik does not register such downloads or at least I’m not able to make it do so (yes, website downloads are tracked fine). Adding campaign designators/IDs to links does not help. I did some tests and it appears that PIwik is not tracking links that do not originate from a website and lead directly to downloaded files - that includes typing/pasting an url (i.e. http://website/file.pdf) in a web browser or linking a QR code directly to a file.