If you don't have a smartphone/internet access - please contact the organisation that sends your vouchers to ask if they can provide an alternative, such as a printed version.
If you have a smartphone but no data when you are out and about - Access the voucher on your phone whilst connected to wifi then head out with it already on your screen. To be extra certain, take a screengrab of the voucher's contents. It should still scan just fine. Don't forget you can also print your voucher.