The best way to export from Autodesk Inventor is by using the Inventor to Sketchfab Publisher by nCricle Tech.
How to install
Download the installer EXE file.
Run the installer.
Launch Inventor.
The plugin should be available in File → Publish to Sketchfab.
How to use
Find File → Publish to Sketchfab. If it's not there, check that the InventorToSketchfabAddIn is loaded correctly in the Add-In Manager.
Once your model is loaded, click Publish to Sketchfab, and a dialog box should appear.
Enter a title, description, tags, and your API token, and mark it as Private if necessary.
Click Publish!
It will export in the DWF format. If you're having trouble, be sure to check your DWF export settings before using the exporter:
CAD software like Inventor does not export textures accurately because it does not use UV coordinates.