This is a publishing question. The idea is: someone with a smartphone gets the card, scans the QR code printed on it, and downloads the PDF of your book off the web. I would have thought that a PDF would be pretty hard to read on your average smartphone screen, but I'm told they are used very commonly for that purpose. A link to an ebook version would make more sense, or not?
PDFs use a fixed layout that doesn't scale with the device or window size, so they're not as friendly for smaller screens. Some people do read PDFs on some phones, though -- some screens are pretty big and some people have good eyes and can read the small page. Also, some of your readers might be using tablets rather than phones.
However, I wouldn't make the QR code link to a PDF or an ebook. I'd make it link to a landing page -- a web page where you can include links to whatever download formats you like and say a little about your book. (These cards sound like marketing materials, so bridge that gap between "somebody handed me this QR code" and "I want to read this book".) If your needs change (e.g. new formats) you can just update that page. Someday it might even contain links to the sequel.
For one example of landing pages, look at the per-book pages in the Baen Free Library.