The Tetra4D plugin converts 3D models from various formats into the two supported model types for PDF (U3D and PRC). Acrobat X Pro can only create 3D annotations from those two file types. AutoCAD files (DWG/DXF) are only supported as 2D drawings and AutoCAD cannot export U3D or PRC, so to move a 3D model from AutoCAD to PDF you must export via a mezzanine format such as IGES or STL, which Tetra4D's plugin can then import.
PRC supports b-rep and tesselated mesh models. b-rep expresses the scene in curves as AutoCAD does, with 'solids', whereas tesselated meshes approximate the model using triangles. Tesselation is useful to prevent someone extracting a perfect copy of the model, but with complex organic shapes the files will be larger. U3D only supports tesselated meshes.