The requirements for documents are minimal. At the border passport control, a border guard (not a customs officer) theoretically needs to present a passport (valid for at least 6 months); a travel voucher or reservation; a completed immigration card; a return ticket; a boarding pass for the flight you arrived on. In practice, everything is even simpler, and you don't need anything except a passport, a completed immigration card and a boarding pass from the flight you arrived on.