If you want to replace tickets download link with QR code(s), both on front-end and in the order completed emails, you can download and install this add-on:
The addon utilizes Google Charts API to generate QR Code images and show them in the tickets table instead of download links. That way, your customers don't need to download multiple PDF tickets.
Add-on is installed and activated like any other WordPress plugin and does not have any settings. It works with Tickera as a "standalone" as well as when used accompanied by WooCommerce via Bridge for WooCommerce.