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.