Pauper is strictly commons. If you want to play commons and uncommons or no rares, try Peasant, which is 5 uncommons allowed in a deck.
Edit: Sometimes things get reprinted at a different rarity for different reasons. If anything was ever printed as common, it's considered common for the purposes of the Pauper format. An example would be Oblivion Ring, which has been printed as both a common and an uncommon, meaning it's legal in Pauper.