You can make a WordPress page only viewable when a user is logged in by installing the WordPress plugin Simple Access Control (Plugin website pr3).

What I like about 'Simple Access Control' is that it's simple and efficient.

You can add code or text that is displayed on the page if the visitor isn't logged in by:

1. Click on 'Settings'.
2. Click on 'Simple Access Control'.
3. Enter text and markup into the field. If you include the word 'login', the 'Simple Access Control' plugin will automatically detect it and turn it into a link to the login page.
   Note: This special generated login link will redirect users to the page they were attempting to view before they were forced to login.
4. Click the 'Save changes' button.

To specify what pages or posts require a login:

1. Click on 'Pages' or 'Posts'.
2. Click the page you want to force login for.
3. Specify settings under the 'Simple Access Control' section.
4. Click the 'Update' button.