I can't find any prices mentioned on the site. Is it completely free?
It's currently $0.99/certificate for early adopters.
I also can't find any mention of what types of file can be used.
Any type of file, we certify the raw bytes of your files, it has nothing to do with the file type.
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.
Certificates are like bitcoins, you cannot reverse a transfer, that's what makes the whole system powerful. In the future, you'll be able to make conditional transfers on Bitproof (based on time for example: let's say this file belongs to XXXX for Y days). Is it a feature you would like to see implemented soon? (If yes, to do what kind of business?)