I found a fix on my iPad Air 2 for some Pages files that did not open after updating to iOS 11 that installed the Files app.
I reset “all settings”. To do this go to Settings - General - Reset - Reset All Settings.
There is no loss of files or data, but things like your passcode, wallpaper need to be redone.
My ‘dead’ Pages files came to life as each folder opened, I hope the same happens for you if you have been having this same problem.