Creating a PDF Format Record
To create a PDF format record you can follow the steps highlighted in this knowledge article. Basically you will be entering in all your company specific information that you would want to appear on a Billing or Purchase Order. This allows all the basic information in addition to adding custom footer content as needed.
Creating Your Own PDF Format
If you want to customize your PDF format, all you need is some basic Visualforce knowledge! To create your own PDF format follow the directions in this article.