I do a lot of historic research using books and periodicals downloaded from archive.org. I download PDF's and save everything in folders. The volume of material that I've accumulated was getting overwhelming, so I found Digital Editions and thought it would help bring some order to the chaos. My first attempt to upload 58 "books" resulted in 27 being completely unreadable with the message "document uses unsupported PDF feature". That's a pretty dismal success rate. What gives?