There is a flavor of 3D PDF that contains nurbs solids, used in Model Based Definition. Its pretty rare I think to run into this kind of PDF outside of industry giants, but not impossible. At the moment, Simlab won't read this type of file.

Hi simon,
You could try saving the file out in a nuetral format other than step. Some programs may be saving specialized "proprietary" stuff in a step, so something like iges would have "just geometry"...( michael touched on instancing, etc.)

If you need help with it, someone here could help you oit also.