With a Numbers document up on your iPhone or iPad, look for the wrench at top right and tap it.
Then tap where it says “Set Password.”
Give your document a password, enter it a second time, and enter a hint. There’s no back-door into a password-protected Numbers document so the hint is essential should you forget the password.
Numbers documents show locked icons when they are password-protected. See below.
When you try to open a password-protected Numbers document on your iPhone or iPad you’ll see a message asking you to supply the password. If you can’t remember it, try again. If you still can’t remember it, try one more time– the password hint doesn’t appear until the third failure.
And that’s all there is to it.