"Pages couldn’t read the file. You can try restoring to a previous version.”

Hi Spnbndr178,

Since reinstalling the Pages app didn't help, see if something in your user account is conflicting with Pages.

You can isolate by restarting in safe mode and testing in a new user.

Test after each step to see if your files open properly.

• Safe mode 

1. Shut down the Mac.

2. Restart and immediately hold down the Shift key.

3. Release the Shift key when you see the login window.

It can take a few extra minutes to boot into safe mode. Your screen may flash or flicker when logging in to safe mode. Those behaviors are expected.

This has more details on safe mode:

How to use safe mode on your Mac - Apple Support

If you're able to open these files in safe mode, shut down again, restart normally, and test again.

• Create a new administrative user, and test to see if this happens there. This link explains how to set up a new user: Set up users, guests, and groups on Mac - Apple Support

Have a great day!

Hi Spnbndr178,

Thanks for reaching out here in the Apple Support Communities. We see that you're having issues opening some Pages files since upgrading to Big Sur macOS 11.

We're glad to help.

You mentioned that the files open fine on your iPhone, and if you restart the Mac, everything works fine.

This suggests that something might be wrong with the Pages app.

We recommend deleting Pages, and then downloading it again from the App Store.

This explains how:

How to delete apps on your Mac - Apple Support

Take Care.