No no you don't need to. From your phone is fine.

I saw somewhere that the conductors put stamps on it one by one, but such an inscription even if I want to buy a Verona - Venezia ticket, that is, it is written in many places

Even if this does need to be printed, yes this can just happen at either a ticket office, or a ticket machine at any station in Austria. Look for the ÖBB ticket machines!

how to find them in Italy

You cannot. In this case I would just do as Thibcabe said and take it on your phone. 

I’ve taken quite a few night trains this year and last year, your couchette/sleeper attendant will just make a picture of your phone ticket. I presume this is an ÖBB Nightjet? In that case it is definitely the case of not requiring a printed ticket. 

thanks, I also thought that he could just take a photo of the ticket