I can't find any prices mentioned on the site. Is it completely free?

I also can't find any mention of what types of file can be used.

Lastly, if I certify/own a document, and send it to someone that I later wish I hadn't sent it to, what power of ownership do I have exactly? Can I remotely make the file stop working for them? Or is it basically the same as a notary stamp that proves to them it is still my file.