Shadows would be "possible" in RPS 3D PDF - but very difficult for us to implement.
U3D, (the underlying format), itself, does not do shadows (as far as we know). If anyone has seen other 3D PDF products which do shadows, let us know.
So, to do shadows, we would have to places textures on each shadowed face which already had the shadows on them. This is "possible", but very difficult, and it would probably create a PDF file which was too large to use properly.
We have seen some approaches which use .AVI movies, rather than 3D PDF to create presentations with shadows. SketchUp can do this with sun shadows already, or you can use IRender to create movies with reflections and shadows from point lights as well as the sun.