You can assign a password to a document so that only those who know the password can open the document.
Passwords can consist of almost any combination of numerals, capital or lowercase letters, and special keyboard characters.
Choose File > Set Password (from the File menu at the top of your screen), enter the requested information, then click Set Password.
Select the checkbox if you want to add the password to your keychain.
If you add a password after you share a document, be sure to notify collaborators.
Note: Adding a password to a document doesn’t encrypt the file.
Change a password: Choose File > Change Password (from the File menu at the top of your screen). Enter the requested information, then click Change Password.
A document can have only one password, so if you change the password when you share the document, that becomes the document’s only password.
Remove a password: Choose File > Change Password. Type the old password, then click Remove Password.
Passwords apply to documents when you share them using iCloud, so your collaborators need the password to open the document. A document can have only one password. If you change the password when you share the document, that becomes the document’s only password.
When you add or change a password, the password applies only to versions created after the password was added or changed. To prevent others from restoring unprotected versions or versions with older passwords, stop sharing the document, add a password to it, then share the document again.