I think a confortable keyboard has to be at least 25cm (10") wide, otherwise the keys would be too small and tight together for practical use.
The Tab Pro 8.4 is only 22cm (8.5") wide, and therefore keyboard cases for that size would be a functional compromise (including the Samsung Tab S 8.4 one)
My advice is to use a quality brand BT keyboard; I use a 20€ Acer IconiaTab one (26cm x 13cm wide), works nicely.
Another interesting, albeit pricier, choice is Microsoft´s Wedge (25,6cm x 10cm), keys are maximized and it should be even easier to type on.
P.S. good article on BT keyboards: