@NicNeely - super appreciate the additional context!
Thank you for doing all that testing - you've actually pinpointed this really well. The issue appears to be that iOS is intercepting the Adobe Express share link and trying to open it in the app (or redirect to the App Store to install it) rather than the browser. Since Firefox is your default browser on iOS, it's likely handling the universal link differently than Chrome does, or something like Safari might as well.
For the QR code itself - rather than using an Adobe Express share link as the destination, try linking directly to the PDF's external URL if you have one hosted elsewhere (maybe Dropbox or Google Drive, for example?). Adobe Express share links are designed to open in the app, which is why iOS is routing it that way.
Hope this helps!!
Rose