Might want to try Quickoffice or Documents to Go. I've not had any problems opening MS office files with Quickoffice (both Pro or HD).
Edit: sorry didn't notice you said free. I think there is a free version of docs to go.
Smart office 2 is free at Samsung App store. Haven't tried it but it says it can open MS office files. It's 9.99 in the play store. Might want to try that our.
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