PDF doesn't open on a very specific iPhone device

This is a very strange problem. Here goes:

I have an iPhone app. There's a button, when you click the button, it downloads a PDF and displays it.

This works on all iPhones and iPads I tried it with. iOS 7.1.1, 8.1.2, 8.1.3 all works.

Except for 1 iPhone. This iPhone is running iOS 8.1.3. When I click the button, nothing happens. no error messages, nothing.

It's an iPhone 5c, bought in Hong Kong. All the other devices I tried was bought in Europe. As far as I can tell, this appears to be the only difference.

We tried a hard reset, we tried deleting and reinstalling the app. We tried installing the app via testflight and enterprise distribution. All to no avail.

Any ideas on what else I can try?