Updated October 15th, 2020.
You can password-protect a Numbers document on your iPhone or iPad. No one can open it without the password. This way, if your iPhone or iPhone is unlocked, your document is still protected. It’s really easy to do.
Note: these instructions are for Numbers 10.2, available for free at the App Store.
Notice the black “Edit” button at the upper right. This is something relatively new: there is a “Reading” mode and an “Edit” mode. (Apple added this because people kept accidentally changing things on their spreadsheets as they scrolled around.)
You need to be in Edit mode in order to set a password. After tapping the Edit button you’ll see a circle with three dots in it, near the top of the screen.
When you tap the circle with the three dots, you’ll get to a place where you can set a password.
You might be thinking “Wait a minute, I saw a circle with three dots in it before I tapped Edit, just left of the Edit button! Why do I have to tap Edit?”
The reason is, you don’t get the “Set Password” option when you’re in Reading mode. See below:
When you’re in Edit mode (by tapping the Edit button), you get the options you need.
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 (the hint should be enough to remind you of what the password is).
You also might want to turn on “Open with Face ID.” I recommend that– it’s very handy. Same for Touch ID, if you happen to have an iPhone with a Home button.
Numbers is really smart about the Face ID/Touch ID thing. Even if you have one device with Face ID, and another with Touch ID, all you’ll have to do is enter the password once on each device and from then on your face or your touch will unlock the document.
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 it will first try to use your Face ID or Touch ID. After two tries and two failures, you’ll be asked to enter the password manually.
You will notice that there’s no keyboard showing in the screenshot! That makes entering the password very difficult. This appears to be a bug. You can make the keyboard show up by tapping a few times in the password box.
If you try the password three times, you’ll see the hint. You won’t see it before that.
And that’s all there is to it.