InDesign Help:
Saving documents for use with InDesign 2.0

You can export InDesign CS documents in InDesign Interchange format (.INX) for compatibility with InDesign 2.0. To open these documents in version 2.0, you'll need to obtain and install the Scripting plug-in and the XML Reader plug-in. You can obtain these plug-ins from the Adobe Web site.

To save a document in InDesign Interchange format:

1. Choose File > Export.
2. For Save as Type, select InDesign Interchange.
3. Click Save.