Seat reservations and app are totally separate. They won't appear in the app and it's not an issue. Whenever asked on the train you show both the PDF (seat reservation) and the app (ticket).

Remember to add each train you'd like to take to the app before boarding. This gives you a QR-code which is your ticket.

Thanks a lot for the answer! I have currently entered the trip I want to take in the pass. when I activate the pass will I have the QR code? thanks in advance, it's my first interrail trip and I don't know how it works

Yes exactly. And no need to activate it in advance either :)

perfect, thanks a lot for all the info! they will be very useful to me