Neither AutoCAD, nor Inventor and Revit can import STL files (stereolithography). They can only export STL files as a part of their "3D printing" functionality.
If you want to use (re-import) an existing .STL file for viewing, measuring or reconstructing of 3D geometry, you can use the free online service STL2DWG running on the CADforum.cz server. This brings an additional alternative to the Inventor internal file converters.
Just upload your textual or binary STL file and this web application generates and return a SCR (script) file for AutoCAD (to save to DWG), or a DXF file compatible with virtually any CAD application.
To load the resulting DXF file into AutoCAD, you can just use Open after you click on the web page link. To load the 3D DXF file into Inventor, use the import wizard (Open) and check its options "3D solids", "Use default file names" and "New part, Wires to 3D sketch". In Revit, load the DXF file with the ribbon function Insert > Import > Import CAD.
Since Inventor 2013 you can use direct STL import (Open).
You can also use the STLin add-on utility for AutoCAD - see www.cadstudio.cz/download, or Autodesk ReMake (with FBX export).