Where can I get my hand histories files?

Detailed instructions are provided at PokerStars FAQ under Hand Histories paragraph.
Only English hand histories are supported.
At PokerStars options choose keep your hand histories for a long time (e.g. 150 days).

Can my hand histories be used against me by other players?

Yes, other players can see what kind of player (passive, aggressive) you are and if you are successful player or not.
But you can be sure, that histories of your hands are available at almost all commerce hand histories servers.
Although a hand histories files generated by your poker application contains your cards, it cannot be used against you, because this information is hidden after uploading the files.

Which poker sites are supported?

You can upload and download only hand histories of PokerStars at the moment. Maybe other sites will be supported in the future.