Open InDesign files from CS5 or later for use in CS3

You cannot save back directly from InDesign CS5 or later to CS3 or earlier. InDesign CS5 doesn't support INX export. Save back to CS3 via CS4, which supports both INX and IDML.

In InDesign CS5 or later, choose File > Export.

Choose InDesign Markup (IDML) as the type (Windows) or format (Mac OS).

Click Save.

Using InDesign CS4, open the *.idml file you created in step 3.

Choose File > Export.

Choose InDesign Interchange (INX) as the type (Windows) or format (Mac OS).

Click Save.

You can now open the INX file in CS3 and save it as an InDesign document.