Dropbox scans user uploaded content pretty aggressively, well beyond what is legally required. It won't always do this in a visible way, either. I know child abuse images will get you silently reported to LEO (as well as anyone who views the link), but I'm not sure about copyrighted content. Personally, I would refrain from storing any pirated songs or movies on Dropbox, since I think they'd be amenable to bulk snitching to large copyright holders.