You will get the reservation as a PDF in e-mail or by logging on to the site you bought them from.

For France, they will generally want to see the reservation if there is a pre-boarding ticket check and possibly both res and ticket (eurail app) on board.

With the app you show the pass, some staff will just scan the QR code, others may want to view the text printed below which has all the relevant ticket information and includes the trains you currently have active for that day, this is the only “page” of the app you display, you do not show the contents of the “My Trip” section.

Thank you @Al_G 

Basically I can store the seat reservation (PDF) anywhere on my phone, right?

As long as it is displayable/viewable on board the train to the ticket inspector who will check the ticket (eurail pass, trips (tickets) active for that particular day) then it will be fine.

Hi again 

I found my answer. I need to print the seat reservation e-tickets for France. 

Reservations for Italy, France, Austria, Switzerland, Germany and the Benelux will be sent to you as e-tickets. You need to print these e-tickets and bring them with you on the train (Source: https://www.eurail.com/en/help/reservations/how-do-i-make-seat-reservations )

Thank you. 

You can show them on your phone too. The vast majority of people do that.

But if you're worried feel free to print them. You always have the phone in case.

@thibcabe Yes I prefer the storing the PDF on my iPhone. I will do that. Thank you for confirming. 

I tend to save PDFs to the Books app on my iPhone. It ensures they’re available offline. 

I’ve had problems in the past with PDFs and QR codes that are definitely visible in my email … until I need to show them and the mail program has somehow decided to try to download it again!

Thank you @ralderton 

On my iPhone, I like using either save my PDFs either in Acrobat or Documents (Readdle) https://readdle.com/documents as I can access them offline even when I’m not travelling to save my data for driving navigation. Yes I hear you; many times the images or whatever would take forever to load in my email program as well.