re: cheap airline question (Ryanair)
@follpvosten While this isn't Ryanair, the Dutch Railways make the exact same recommendation; in their case it's because the app disables the NFC functionality while scanning the ticket off the screen, as they use dual NFC/optical reader units, and they don't want you to accidentally activate the digital wallet on your phone and such.
The need for an app can be avoided in that case by turning off NFC manually when scanning the ticket. I don't know for sure what Ryanair's reasoning is (and whether it's just a ploy to get you to install the app) but perhaps it's something similar?
re: cheap airline question (Ryanair)
@joepie91 @follpvosten
I have a similar experience with other cheap airlines (and even some non-cheap ones). a PDF will read less clearly than a printed paper, but usually it's also fine.