It depends on the operator, but you must often print the voucher.
If your trip has an e-ticket sign, having the PDF attachment on your phone will be enough for traveling. Otherwise, you must exchange the printed voucher for the ticket or have it on hand during the trip.
Please carefully check the website, confirmation email, and PDF information. In some countries, finding a place to print things may be challenging, but your hotel will often be a solution.
Some operators may refuse boarding if you don't have a printout. If you don't follow their procedure, you will not receive a refund.