I find extraordinary that numbers for ipad has no password protection and that this has not been corrected in spite of repeated questions about it: i) it is perfectly legitimate to expect it, ii) it exists on the original program on the computer ( and therefore I was fully expecting it as a feature on the ipad when I bought it) and iii)other apps, which cost less have it so it must not be that complicated.
Not sure when it was implemented, but there is a way to require a password to open either a Pages or Numbers file on a Mac. (Not sure about the iPad version.)
The option is at the bottom of the Document Inspector page of the Inspector pallette.
iOS iwork ( pages,numbers,keynote on iPod/iPad/Iphone ) does not have this - in any version. That's what has been discussed here .
Out of curiositrois, I saved a test spreadsheet as password protected on my Mac version of Numbers.
I then used iCloud.com to upload it, and accessed it from my iPad.
Sure enough, when I tapped it, I get a password entry box on my iPad!
Not so fast...
After typing in the password the first time, it stays unlocked.
It also modifies the file so that if you download it on iCloud, it is unlocked there as well.
Here's another way to do it, but it's pretty manual.
Create your Numbers/Pages/Keynote document on Mac OS X without "Require password to open".
Instead of using iCloud, save your document to either Dropbox or Box (they both have sync apps on OS X so you can just save your file on an ordinary folder in Finder).
Grab your iPad.
Open Drobox or Box (download them from App Store if you don't have them).
Be sure to set up a Passcode Lock, so only you can access your files.
Find your document and lookup "Open In", and then choose Numbers/Pages/Keynote depending on your file.
Work on your file and when you're done, use Share and Print \ Open in Another App and choose the original Numbers/Pages/Keynote format.
Ask to open in Dropbox or Box. This would create a copy in that app.
Remember to delete your document from Numbers/Pages/Keynote in your iPad, because its visible to anyone.
Hope this works for you.