Drawing files that have DWF underlay attachments can be plotted or published as DWF files. DWF underlays are tracked and managed when you eTransmit. Furthermore, DWF underlays are not lost when you save to previous drawing formats.
When a drawing file containing an attached DWF underlay is plotted or published to a new DWF file, any visible DWF geometry is included in the newly plotted or published DWF file. However, none of the layer data from the original DWF attachment gets published with the new DWF file.
If you save the drawing to a previous DWG format, the DWF underlay is not going to be displayed when the drawing is opened in an older version of the program, nor will it be displayed as a proxy object or a proxy frame. The DWF underlay will display again when you reopen the drawing in the current version of the program.
DWF attachments are tracked and managed when you use eTransmit in the same way raster image attachments are tracked and managed.