Seems like an unnecessary device function for AR to have access to. What does it supposedly use it for?
And, I'm more than a little irked that the app's support email address simply auto-replies with the directive to go to this forum, which requires registering for a new account/login. That's garbage. You should be able to get support without creating another online account.
In Adobe Acrobat for mobile we have a feature to sign PDF by selecting an image file directly on the device or clicking a picture using Camera of the device.
Open PDF in Adobe Acrobat
Tap on Comment at the bottom Center of the screen
Under Comment Tool-set tap on Signature icon at the bottom right
This will take you either of the options as described above.