Not sure I understand the question completely, but you can "Save As" a PDF and click the button 'Save As 3D PDF". Or you can save as any number of other formats as well.
SolidWorks does not export 3D DXF or 3D DWG. You will need to export as another format that your version of AutoCAD can import, then export from AutoCAD as dxf or dwg. In theory an old version of ACIS (.sat) would work best, as AutoCAD is based on an older version of ACIS, but I usually don't have good luck exporting ACIS from SolidWorks.
To export from solidworks 3D to autoCAD 3D we alway use .SAT version 7.0 with no problem. I have exported assemblies with as much as ten thousand parts in it with no issue but the fact that I could go home, have a good night sleep and drive back to the office in the meantime.