DRF is the file format for VIZ Render, a rendering tool formerly included with AutoCAD Architecture. The DRF file type is similar to MAX files from previous versions of Autodesk VIZ.
This file format is available only when you use the Application menu 
Open command. All DRF files must be saved as MAX files in 3ds Max. Likewise, DRF files cannot be imported or merged into 3ds Max scenes.
Note: You can open DRF files using drag-and-drop functionality.