I will review the documentation to see if I can follow.
The PDF pulls based on datamapper; possible choice of 800 PDF’s - all individual names and page counts.
FYI. Scripting should to the trick for now. In OL Connect 2019.2 we will introduce a Section Background Wizard to achieve this without scripting. The wizard let’s you specify a base location (Images folder in the template, a folder on disk or a base url) and construct the final location by selecting a data field.