Unfortunately you cannot download to a SD Card on a standard android tablet running anything lower than Marshmallow. All prior versions do not allow for downloading to a card.
In Marshmallow it can be done, but there are limitations to that. One, your devices manufacturer has to have not blocked the ability (some companies, it seems, have blocked downloading to the card even in Marshmallow) and, you have to have a blank card that will be formatted when set to "Internal" rather than "External". I say you need a blank SD Card because when it is formatted to work as "internal" it will erase all data on the card. Also the card will no longer be readable by other devices due to the encrypting done when it is formatted "Internal".
So, if...