To open an InDesign CS3 document in InDesign CS2, you must install the appropriate plug-ins in the previous version of InDesign and export the InDesign CS3 document to the InDesign Interchange (INX) format.

You can obtain the compatibility plug-ins for InDesign CS2 by choosing Help > Updates and following the prompts. You can also obtain them from the Adobe website: Choose Support > Updates, and go to the InDesign Downloads page.

Be aware that content created using features specific to InDesign CS3 might be modified or omitted when you open the document in a InDesign CS2.

You can open the .inx file in InDesign CS2 to convert it to an untitled InDesign document.