I would like to use thumbnail versions of a file act as a button to the full version of the file. I assume there is a way to do this but can't seem to find any documentation that describes how.
You can upload and host your images, including PDFs by following the steps outlined here.
You can then add these images to the body of your page and create a hyperlink as needed so that your site visitors can download the file.
Complete and current information on how to use NationBuilder is available in the documentation section.