Author here. It is free for 30 days, and I think most people who try it will find that it's worth the price. And if not, I'll think about bringing it down when the trials start expiring and people don't convert. For what it's worth, there are desktop apps on the market that do the same thing, but not as well, and cost significantly more.
Textcelerator doesn't have PDF support yet, but it's planned for a future version. Textcelerator runs anywhere Firefox or Chrome des, so Ubuntu is and will of course continue to be supported.
Also I am looking for a Pdf speedreading tool for Ubuntu. Would appreciate any recommendations.