That depends on the long distance bus provider with who you would like to travel. There are different options to show your ticket, which you received per Email. So therefore that bus ticket does not have necessarily have to be printed. With some providers it is sufficient to show the QR-code on your cellphone.
In exceptional cases it is sufficient to show your ID card.
Please note: We recommend to always print your ticket – just to be on the safe side. If your cellphone battery dies, you will be able to ride along anyway.