[Nextgrid]

Does the file have to be generated on the fly, or does it last for a reasonable amount of time? In the latter case wouldn't a printed copy work, or just setting your lock screen background to a screenshot of the document zoomed in on the barcode.

Alternatively, on iOS, "Guided Access" can be used to lock the phone to a particular app (let's say the browser) and even prevent certain parts of the UI from being tappable.

You can bring up the document, activated Guided Access (it has its own separate passcode to disable) and then hand the phone over. From there they can either remain in the constrained app, or if they restart the device they'll get the standard system passcode before they can proceed further so it's fairly safe to prevent the device from being misused.