My client has older materials with QR codes that took students to a PDF he had hosted on his old Wordpress site. Can I change the link to that PDF that I now have on his new Webflow site or will he need to make all new QR codes?
If the link was taking users directly to a PDF file (ex: yoursite.com/files/my_document.pdf) then you’ll probably need to update the QR code. If it was taking users to a page where the PDF document is hosted—which I’d argue is a better solution long-term anyway—then you should just be able to redirect that link to the new URL fairly easily.
I’d suggest that the QR code is updated either way to lead to a URL (ex: yoursite.com/pdf-guide) and then control the specific location of that URL as needed with a redirect. This way you can choose to link to just about anything (landing page, direct file, signup form, etc) and have better control if file paths or file names change down the road 
Thanks Mike, that’s the best route, make some new QR codes. But if they MUST use the same old QRs, where would they redirect the link? Would I do that or what they need to do that where ever they got the QR code made?