I did a quick scan of their "terms" and there wasn't anything that resembled a privacy policy, so they may be keeping and selling all the documents they are freely converting for you, so obviously, don't use a service like this for sensitive or confidential documents you wouldn't want anyone else to see. :)
I've used that site many times. It's one of many cool "web2.0" (I hate that term) apps that I've found on del.icio.us/tag/web2.0
