I know that I will get hammered for this, but this is my opinion. I completely agree with the Open Source Movement. Open Source was supposed to be where you could either pay less for software or get it for free. It seems now that the skewed view of Open Source has crept into the Joomla! community with this plugin.
They contend that you should have to pay for their documentation of their "free" software - but the software itself is "free". I am sorry but if you make software and tout it as being "free", the documentation should be free as well. Either you make it a paid propreitary product, or you offer it for free - no strings attached.
I am a classically educated software engineer and I have no problems with paying for products put out by the big corps like Adobe because that is what they do and they do not claim to have free software.
I am not going to spend 35.00US for a pdf file when I can go out and buy a real book for that amount with a pretty cover and a free tacky-looking bookmark.
With all this said - they have a very good extension here. Its functionality is unsurpassed and I love the customization - if you can figure it out. You will need to play with it for about three hours before you get the hang of it - unless of course you decide to pay an outrageous amount of money for a limited time PDF file.