Use STL Export to customize and export your Revit model to an STL file.
- Open a 3D view.
Note: The exported geometry will be based on the current view. Geometry exported will reflect the visibility and graphics settings of the current view.
- Click the File tab
Export
CAD Formats 
STL.
- In the STL Export dialog , specify your STL file options.
- Click Save to create the STL file or click Cancel to close STL Export dialog.