It probably depends on the airline but I'd guess most will take an electronic copy. While my Emed tests were duds I did get a PDF from the local test place I used in Athens and uploaded it to United directly via their app a couple of days before. They approved it within minutes (they do some manual backend review) and I was able to get my boarding pass today no problem when I checked in at T-24. The United app shows that I have met all the requirements I need for reentry to US. I have a print out but I doubt I'll need it. I will note that the WiFi at hotels can be iffy. When I had my session going with Navica it dropped me once (before I started the test and I had to reenter the queue) and during the two (dud) tests I attempted there were several 10-20 second intervals where the connection wasn't working but it did come back with the same person I had been dealing with. I have no doubt it likely would have worked well enough to get through the process had we got to that.