I need to explain a little better because your letters casts shadows on themselves. So my previous advice isn't quite right.
Suppose you have a ball sitting on a table. A shadow pass with the ball visible will produce a shadow with the ball shape cut out of it (i.e. shadow looks like a crescent). This method is nice because you can put that layer right on top of your ball diffuse layer. To soften the shadow though, you'll need to use a brush or something to blur just the outer edges of the shadow.
A shadow pass with the ball invisible will produce a fully rounded shadow. Having a fully rounded shadow makes it nice in compositing because you can apply a blur to the whole layer and soften the shadow very easily and quickly. But if you try and blur the crescent shadow, then it won't work because the shadow will look like it's pulling away from the ball because the area where the shadow and the ball meet will be blurred.
With that said, I think it will be easiest for you to render out your scene with 1 shadow pass and with everything visible, but just make sure your shadow's edge softness is right before you render. Then just place your shadow layer on top of your diffuse layer/s. You can still adjust shadow opacity in post production and such
Don't be satisfied with what you can do but rather strive to do the things you can't do!