Pages can not be password protected, or is there any built in password protection for files in Pages.
Without a jailbreak (not available for iOS 6 at this time) you can not password protect apps at the system level, only the entire iPad.
That is not to say that apps can not implement their own password protection. There are several choices for storing protected files. However I am not aware of any word processing apps that do this; at least not any good ones.