If it isn't current, it would improve our response logic by seeing current hardware, operating system, and Pages version in your community profile.
Are you able to successfully open other iCloud resident documents with Pages?
If you want this document locally, open Pages to its file dialog where you can see this document in the iCloud panel. Press and hold the option key while dragging the file you cannot open to your desktop. Now you have a local copy and have eliminated iCloud from the file open equation. If Pages still freezes while opening this local document, you have another issue.
On IOS and OS X, I would revisit the iCloud settings/preferences panel and reverify that you are signed into iCloud and that Documents & Data was still selected. If these are already enabled, you might try turning them off and then back on. Reboot your iPad and Mac too. See if you can open that pesky file on iCloud after these steps.