Want to set up password protection for files you’ve created in Pages, Numbers or Keynote in macOS Sierra? It’s easy.
From Pages, Keynote or Numbers, open the document that you want to secure with a password. Go to the File menu and choose Select Set Password. Enter a password and a hint. That’s all there is to it.
As you set a password, a small lock icon will appear on that particular document indicating that it's secured with a password. The next time you need to access thedoc on any Apple device or iCloud, you’ll be asked for a password.
(This how-to is based on my experiences and info on Apple's support pages.)