It seems like your client already has a bunch of printed QR code that they wish to scan and open a pdf from nextcloud

Have you considered a USB/Bluetooth QR code scanner that would mimic a keyboard to enter the scanned codes into a nextcloud search bar?

If you have the URL to your file, can you not just generate a QR code that opens that URL?

Are you looking for something to generate QR code for every file you have?