They just need to be able to scan the QR code. As long as their scanner can pick it up you're good. Worst case scenario is they have to enter the reservation number or look up your name if they can't scan it. I always have a paper backup except for the time when I made my purchase minutes before I got to the station.
Do you have an iPhone or Android phone? If you do you can display the code if you've got an Amtrak account and are logged in on the phone's application.