Note: You can not make uploaded files (pdf, word docs, images, etc) private, ever.
Even if you place an uploaded file on a "private" page the file itself is not private.
If you upload any file to the site it will be viewable by the public and may be found by search engines such as google. Never upload any confidential file to the website.
To make a page private follow the steps below.
Click "edit" on the page you want to make private. Scroll down until you see a checkbox labeled "Make this post private." Click the checkbox.
Click "save" to commit your changes, and view your page. Your page should b private! Of course, since you're logged in you'll still be able to see the page. To check and see if the page is really private, log out and try to view it again.