Hello Topher,
Thanks for reaching out to the Apple Support I understand you've been receiving an error when you go to open a document in Pages.
Since you have the document stored in iCloud, open Safari and sign into the iCloud website. See if you can open the document from Pages in iCloud. This will help to isolate the issue. If the document will open in Pages in iCloud, we know the document is not corrupt.
Next, work through these troubleshooting steps, if you haven’t done so already. This will help us to isolate the issue further with this document and the Pages app:
1 - Quit Pages. If it will not quit, follow these steps to force it to quit: How to force an app to quit on your Mac.
2 - Restart your MacBook Pro: Go to the Apple menu in the top left corner of your screen > Restart. See if you can open the document in the Pages app. Try opening the app first > File > Open > select the file. If that doesn't work, open Finder > iCloud Drive > Pages > select the file. If that document isn't opening, try another document to tell us if it's isolated to the one document or any Pages documents in iCloud Drive when opening them from the app.
3 - If the issue persists, restart in safe mode and test again: Use safe mode to isolate issues with your Mac.
4 - If still occurring in safe mode you’ll need to create a new user and test it out again. This article will show you how: How to test an issue in another user account on your Mac.
Let me know the results.
All the best!