They can work but right now, traffic to PDF's on the site are only a small fraction of the activity.
Videos, audios and articles work way better and are 100x times easier to manage.
PDF's do work however for the $19 programs.
When the ebooks are open, people do access them as well but creating them is not as much fun as recording a video or audio.
here are some stats to help you see what's happening:
overall video views/month (that's all video sites estimates including youtube: 100'000-150'000 views/month
overall audios downloads: 40'000-60'000 downloads/month
over PDF's downloads: around 1000
This is it! Numbers speak by themselves.
Yes, I have many more videos and audios available too of course, this is simply to give you an idea...