Mostly you can just show the reservation on your phone, if indeed anyone wants to see it.

With some, like Eurostar, you can add the reservation to your Wallet,

I don’t know if there are any operators that would insist on a paper copy.

And yes, it’s very annoying that Eurail isn’t a one-stop shop for reservations. If they could fix that, it would make things so much simpler!

I’ve never had a problem with using a pdf on a phone. However if using a sleeper you normally  need a physical ticket. The ticket office should  be able to print one for you but I can’t remember if I’ve ever had a sleeper on a pdf. I normally collect them from the ticket machine or buy them at the ticket office

I've used the Nightjet many times with a printed PDF reservation. IIRC ÖBB request printed reservations for international journeys and it does work best because the coach attendent still expects to collect the reservations.

This is a regular bone of contention and there is no definitive answer that covers all trains. 

However as more and more operators move to pdf reservations their inspectors will accept a pdf for reservations. However if like me you like to be prepared for anything I always print out all my train, hotel and flight reservations and take them with me. If nothing else it saves me swapping apps when inspectors want both.

If you get reservations in stations you always get a printed version, unless the operator sends it digitally to your e-mail or text, when obviously you can show it onboard.

With the exception of sleepers I think a pdf is now widely accepted on trains with mandatory reservations.

For trains with voluntary reservations you only need the details to find your seat and to eject squatters.

Hi,

I had the same concern about printing emailed reservations while travelling with Eurail. I found a solution by visiting a local printing company called Packaging Mart in the UK.

They helped me print my reservations, so I didn't have to carry a printer with me. It was convenient and allowed me to show the email instead of a printed copy. While Eurail is a great resource, using local printing services can be a workaround for this issue.