Well, it depends on the bus company.
If you've already bought a ticket or voucher, you'll find all printing and boarding information on the purchase confirmation email we sent you as well as on your ticket.
For example, this confirmation email clearly asks that the tickets be printed.
If you haven't purchased a ticket, this information will also show up on the checkout page, in a box like this:
If you have a voucher instead of a ticket the same protocol applies: most companies require that you have a printed voucher in order to exchange it for a ticket at the bus operators counter. With that ticket you'll be able to board the bus.
I don't get it, other bus companies don't force me to print my ticket. What's up?
Bus companies' policies vary greatly with regards to ticket printing. Some bus companies **do** require that your ticket be printed for you to board the bus. However, an increasing number of bus companies now offer e-tickets or allow you to board by showing your ticket to the driver on your mobile phone or offer "Get a station" where an agent will print the tickets for you.