Near the bottom of the screen in the Bill summary section is a View bill (PDF) link that displays a PDF copy of your current statement. If you want to view a previous month's PDF statement, click the View statements link at the top of the page. Then select the desired billing cycle from the Billing Period menu. You'll see a View Bill Details (PDF) link that will display a PDF of the selected billing statement.
If you haven't already signed in to your Cox account, you'll be prompted to enter your Cox User ID and Password when clicking any of the the above links that reference your personal data. You must already be signed in to view a PDF statement using the View Bill Details (PDF) link. You won't be able to create a one click shortcut to a previous month's statement, but you can access PDF statements for any of the last 18 billing periods through the View statements webpage.