It depends on the operator, but yes, you need to print the voucher in most cases.

We do recommend printing the voucher even if the operator accepts the screenshot. The first page of the voucher contains a map, the address in the Thai language for your taxi driver and the contact number for your booking, which could help you in the emergency situation.

In case you are traveling by the Thai railways, there is no need to print out the voucher.
You can show a screenshot or tell us your booking number when you pick up the tickets in our office or in Bossotel in Chiang Mai.

SeaTran Discovery Ferry requires a printout of the voucher only on few stations in the Western part of the country. Their main counters accept screenshots as well.