I found this thread rather late, but here are a couple of notes (re: Pages and iOS current as my writing):
You can password protect a Pages document saved in iCloud, and if you try to open it in iOS, you'll have to enter the password.
But if you edit the document, the password will be removed from the document. That applies to both iOS and OS X.
The warning says that opening it will remove the password, but I have not found that to be true. It's editing that does the deed.
If you need to edit, you can choose to open a copy of the document, leaving the original unchanged. But then you'd have to password protect the copy on your Mac.