You could use QR Code for files in the classic experience.
1.Go to the document library -> Click Return to classic SharePoint in the lower left corner.
2.Select the file -> Click on the tiny phone icon -> This takes you to a page with a QR code generated for the document URL.
Reference:
https://www.sharepointdiary.com/2016/12/how-to-generate-qr-code-in-sharepoint.html
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