We've all been there - a UWP application isn't working quite right. When you're pretty sure that it's not your fault and it's probably not the application's fault. Something has clearly come unhinged behind the scenes. Happily, with a caveat, it's trivial and quite quick to reset the application 'in place' and be up and running again.
Now, obviously, if something's not working then it must be a bug of some kind. Either in the app or in the OS, but it's not always practical or convenient to down the rabbit hole of blame. Sometimes you just want to try the quick fix and be up and running again. Which is where the Windows 10 Mobile (hidden) 'Reset app' function comes in.
In fairness, you could do the same by uninstalling the application completely and then reinstalling it. But that takes longer and you have to find it all over again in the Store and it's just... messy. So here's a walkthrough of resetting an application in place.
In my case, Perfect PDF Reader wasn't opening documents from OneDrive and I couldn't see what the problem was. My thought was that it was ages since I installed the application, the OS had been through multiple monthly updates and maybe it was time for the application to be digitally 'reacquainted' with the OS.
Head for Settings/Apps/Apps & Features', it'll take a moment to populate the list if you have lots installed (like me), but five or so seconds later you should have a breakdown of every application on your Windows 10 Mobile device, along with a helpful readout of the amount of storage their code and data is occupying; (right) tap on the application in question and you'll see an 'Advanced options' link in dark blue. Tap on it.
In this semi-hidden dialog, there's a happily much more obvious 'Reset' button. Essentially this removes everything but the app's own code, so it will behave as a freshly installed application and can built its temporary and account files as needed. And will hopefully then work for you!