One of our customers wants to use QR codes in their PDF receipts. It's to check in at an event.
1D Barcodes are much easier to implement than 2D Barcodes because you only need to display a number (or string) with a font.
If you want to implement 2D barcodes, you need to rely on a service to generate an image which you can Base64 encode and display in your form using a thumbnail widget. You could use the Submission Completed Processor service extension point to inject the Base64 data into the form before the receipt is generated. There are many QR code generator libraries and services which you could try to leverage.
Our client is asking if you have a preferred supplier/software for the barcode scanners?
I passed this onto our customer and they said they'd be happy with a 1D barcode. Is there any documentation on how to use this widget?
Adding fonts is still an advanced configuration, there is no widget, you will have to test and formulate a solution, based upon some documentation which may help.
If you search the kb for "fonts", you may be able to find some useful material such as the following:
I can't attest to this being a solution, only a possible path to attempt.