This app does what it says it does: it syncs your DriveThruRPG library. And that's really about it. It doesn't provide a way to open the downloaded files in your choice of PDF reader, it doesn't provide a way to use any of the modern tools for file management that debuted in iOS 11. It uses the built in minimal PDF viewer tools, providing a poor user experience and not allowing any kind of bookmarking, annotations, or other rich interaction available in a dedicated PDF viewing application.
As such, users are still better served by downloading their documents directly from the site. I have a specific PDF reader that I read all my PDFs in, which supports bookmarks, annotations, editing, etc., and integrates with my iCloud Drive so I can store files in one place and read them on my iPad and my Mac. The DriveThruRPG app doesn't support any of this.
This is a 5 star app if judged solely by what the app sets out to do, but a 1 star app if judged by how I want to use it. The files we get from DriveThruRPG are already DRM-free; please let us do reasonable, modern things with them in your app!