You can use a Eurail Global Pass Mobile version only on the Railplanner App on iOS or Android devices. There you can also activate it. 

There is also a Paper version, that have to be sent by postal mail or you buy it at a Ticket Office in Europe. 

You can buy a mobile pass on any device you like. After the purchase, you will receive a confirmation mail with a pass number. The pass number will also be visible in your Eurail account. The pass will only be linked to the device on which you do the activation.
Also see:

https://www.eurail.com/en/eurail-passes/eurail-mobile-pass/getting-started

It is advised only to activate your pass on your first travel day. You don't need to activate to book reservations.