Download raises either the Speical Attack or the Attack stat, depending on the foes defensive stat.
Download raises either the Attack stat or Special Attack stat by one stage depending on the foe's lowest current defensive stat (i.e. it will raise Special Attack if the foe's current Special Defense is lower than its current Defense stat). If the foe's Defense and Special Defense are equal, the Special Attack stat is increased. During a Double Battle, Download will add the two opposing Pokémon's stats and calculate the lower stat between both. -Bulbapedia
So, it will raise your Special Attack if the foes Speical Defense is lower than the foes Defense.
It will raise your Attack is the foes Defense is lower than the foes Special Defense stat.
If both stats are equal, the Speical Attack will be raised.
Defense > Special Defense = Raises Special Attack
Special Defense > Defense = Raises Attack
Defense = Speical Defense = Raises Speical Attack