Which country ? Usually a PDF is fine.

Seat reservations in Eastern Europe are sometimes only available on paper through eurail.com but you can often make them online elsewhere (no or less fee).

In the email that they sent me, it says that it is necessary to have it printed on paper.  the trip is from italy to switzerland

Yes I know but staff won’t care if you show the PDF (per my experience at least)

Ok! Show the PDF from mobile?

And... Do you know if you have to validate the pass before boarding each train?

Yes show PDF from your phone.

You need to add each train you’d like to take to the app before boarding : that way you get a QR-code which is your ticket. Nothing else to do

perfect! I did that already! at least the first trip

Thanks a Lot!!