Link is broken, and Acrobat doens't have the auto convert function any more 😞
I have found a free 3D application "MeshLab"
With Meshlab you can convert many 3D formats in the famous .U3D format !
I realize this is an old thread. But since acrobat pro supports 3d objects, the only way I found to use u3d files is to use photoshop CS5, that format was removed for CS6 and CC I don't know why, you would think they would have kept it, until acrobat supported other formats.