Hello and welcome to Apple Support Communities abr1111,
I see that you're unable to open a document in the Files app. I can understand your concern, and I'd be happy to help.
Take a look at the articles below with some information that can help with what you've been experiencing.
Use the Files app on your iPhone, iPad, or iPod touch
View files and folders in Files on iPhone
If the issue remains, respond back with some more details. You stated that document tries to or opens in another app. Which app is it opening in? Is the specific document that you're trying to open in Files, one that you've open with Files before? Have you made any recent changes to your device? Which version of software are you using?
Thank you for using Apple Support Communities.