This is a bug with V9. Here's the answer I got when posting this same problem on the DWF Viewer discussion group:
Reply From: Jeffrey Klug \(Autodesk, Inc.\)
Date: Sep/21/04 - 21:25 (GMT)
Re: Viewer / Autosketch 9
This is not a valid DWF file. If this truly came from Autosketch, you
should report this as a bug.
The DWF specification mandates that the first 12 bytes of any valid DWF file
(for any version of the format) must be "(DWF Vnn.nn)", where the nn.nn
represent the version. The file you posted contains "(W2D V06.00)", which
declares this to be an internal 2D resource, not an actual DWF file.
Resources of this type are found within DWF files but are not "free-standing
I reported this bug weeks ago but have not heard of a fix