We recently got an iPad Pro 12.9" and wanted to show off our application on the amazing huge screen. But it looked really terrible 
For Retina screens on iPad Air and iPhone you have to set r.MobileContentScaleFactor=2 in the BaseDeviceProfiles.ini in order to render at full resolution, but that wasn’t happening at all on the Pro. I got in contact with support and found a few issues.
First comment was that Pro 12.9" should have the scale factor set to 2.67. But the real issue is that Unreal Engine has never supported device profiles for iPad Pro because of some mismatches. The device is looking for settings titled “iPadPro_12.9” and “iPadPro_9.7” In the .ini device profiles for the last several engine versions the Pro sections were labeled “iPadPro129” and “iPadPro97”. This should be fixed soon (writing this at time of 4.14.0), but any current and old projects need to have the Pro sections of the BaseDeviceProfiles.ini edited with the punctuation.