If you can open that password protected Pages document in Pages 7.0 (10.12 or later), then you can use the File menu : Change Password…, enter the password, and uncheck the Remember this password in my keychain item. If you open that document in Pages v5.6.2 (El Capitan), that italicized selection is unselected, and you would have to remove, and reset the password again. Manually save the document after the changes.
There is no keyboard shortcut to set/unset the italicized checkbox in the Set/Change password panel.
Your iPad is a different device, on a different operating system. As I mentioned earlier, unless you are syncing the keychain via iCloud between your Mac and iOS devices with the same Apple ID, then the iPad is going to prompt you for that Pages document password.