PDF ProCSS publishing is based on 'Resolved XML'. Resolved XML output is a linearized version of the Published XML output. When you publish to XML Author-it generates a complete set of objects that allows you to restore the book in another Author-it library. That means that all variants and related objects (such as templates, styles and media) are included. Publishing to Resolved XML asks for variant selection and will only export the objects that are directly related to the book and the selected variant.
Resolved XML can be used to create a PDF or other output formats, based on XML.