Train tickets on phone on Swiss Federal Railways?

Things have changed since 2017. Ticket on phone is fine, quoted from this web page:

https://www.sbb.ch/en/travelcards-and-tickets/buying-options/online.html

"Tickets can be purchased directly at SBB.ch and displayed on your laptop, tablet or smartphone – even without the app."

"You can purchase your ticket up until the departure of the public transport option you have chosen. You must be in possession of a valid ticket at the time of departure of the public transport option you have chosen. The order must be fully completed before the departure of the service and the ticket must be available either in your mobile device’s application or in printed form. If not, you will be required to pay a supplement in accordance with Tariff 600.5, point 30.00 (in addition to the fare)."

The SBB (Swiss Federal Railways) website states (see "Ticket checks")

So that the ticket you have purchased online can be checked, you must show the printout with a valid ID document. Alternatively, you can also show the ticket purchased on the SBB Mobile app.

The latter option is however only available if you bought your ticket on sbb.ch and not on some other travel website.

After downloading the SBB Mobile app and logging in with the same account you used to purchase the ticket online, your ticket should automatically appear on your phone.

Just presenting the PDF on your phone does not seem to be an option, so you will have to print it out if you don't want to install the app or didn't book directly with SBB.

Edit: Referring to Jonathan's comment below: The website, in the FAQ section for online tickets, explains what you should do if you have printing issues (which is also what you should do if you don't want to print out the ticket):

You have 3 options:

• Option 1: display the ticket in the SBB Mobile app.
• Option 2: you can have your ticket sent via MMS on your mobile phone. However, you can only obtain this so-called MMS ticket for selected routes.
• Option 3: rectify printing problems.In order to view and print the ticket, you must have a PDF reader installed on your computer.

That means they really don't like the idea of you showing them the PDF file on your phone (not even as option 4 after "fix your printer"), so it does not seem very advisable. However, I don't have any personal experience about whether it would be accepted in practice.