Before you start modifying your PDF CSS, get insight into the default PDF CSS:
/@style/common/prince.css.https://<your-site.com>/@style/common/prince.cssCheck for styles you can modify to fit your own styling.
Be aware that content: <value> adds information to the PDF. If the content: <parameter> includes a flow value, then there are elements that are passed to the PDF that cannot be modified, only overridden.
PDF CSS does not have a Revision History. Changes are final and cannot be reverted
To customize styles:
Added styles must be supported by Prince. For CSS formatting help, refer to http://www.w3schools.com/css/default.asp.

Review sample styles to start customizing your PDFs.