Hey there, kaige_g.
After reviewing your post, it appears that you're having a problem opening files from the Files application on your iPad. They open fine when using your iMac on macOS Catalina. You don't receive any message that would help locate the issue, and we're glad to help with this.
Update your iPhone, iPad, or iPod touch -- Make sure that the iPad you're using is up to date. The current iOS version is iPadOS 13.6. If your iPad needs updating, be sure to create a backup first: How to back up your iPhone, iPad, and iPod touch
What’s new in Pages for iPhone, iPad, and iPod touch -- Also make sure that the iWork apps on your iPad are also up to date.
Set up iCloud Drive -- Another thing to check is that iCloud Drive is turned on by going to Settings > [your name] > iCloud.
Restart your iPad -- If everything is updated and settings are turned on, restart your iPad and see if you can open a document.
Let us know how that goes.
Take care.