I remember trying to get into PDFs/text-files when I was a 'young adult' and never could. The thing that really bugged me about ebooks was the tediousness of keeping up with where I was when I took a break. If acrobat had a bookmark function back then, I never figured it out. There was also the restrictive nature of having to have a laptop or be sitting at a PC (before cell-phones and kendle stuff) to read. Kids don't really have those issues now, so I would have though electronic format would be more popular. It's a pretty interesting phenomenon.