How to lock an iWork file (Pages, Keynote, Numbers) Document with a password?

The best way to do this, as far as I know, would be to make an encrypted disk image using Disk Utility. Found in /Applications/Utilities/Disk Utility.app

To do this, open up Disk Utility and select File>New>Blank Disk Image… and select a name to save it as, select a name for the volume when it is mounted, select a size for the volume, leave the format as is, and – THIS IS KEY – select encryption and select either 128 AES or 256 AES, either will be fine, but 256 is more secure, obviously. For partition map, select none, and finally, for Image Format, select either Read/Write Disk Image or Sparse disk image, the difference is that a sparse image expands as you put files inside it. The R/W Disk Image is automatically the size you chose for it when created.

Once you’re all done, select create and it will ask you to choose a password and it will mount it on your desktop, then you can put your files inside. Unmount the volume to lock the disk image again, the next time you open it, it will ask for your password, unless you saved the password in your keychain, in which case it won’t.

Hope that helps!

It says there not enough free space to put anything in.

Try making it a larger size, though I can’t imagine an iWork file being very large.