I have found that adding copious amounts of bookmarks to large PDFs is a huge boost to their usefulness. Unfortunately, the PDFs from DriveThru are locked so you can't add bookmarks. How to get around this problem is left as an exercise for the reader. Here's an example of how my PDFs look, with bookmarks:
It would be awesome if Mongoose would add bookmarks like this to the PDFs so that people don't have to work around the protection in order to have a profoundly more useful file. It only took me about an hour to add all the bookmarks.