Yes, in general, bus tickets are e-tickets. After a successful booking, you will receive booking confirmation immediately by email. The booking number you have to show to the bus operator. It helps to print the PDF file and show it to the employees/driver. Some bus operators, e.g. Panthip, insist on having a printed voucher.
For some operators passengers are required to furnish the following at the time of boarding the bus: a copy of the ticket (a print out of the ticket or the print out of the ticket e-mail) and proof of identity (Domestic Smart ID Card, Driving license, Student ID card, Company ID card, Passport, etc.).Failing to do so may cause the passenger not being allowed for boarding.