If you need to backsave an InDesign CS4 document for use in InDesign CS3, you’ll find that backsaving is not available through File > Save. Instead, you must export an InDesign Interchange file (File > Export). In order to open this file from the future, the users of InDesign CS3 must first update their software to the most current version. If they’ve let all automatic updates install during the life of the product, they’re current. To check if software is current, choose Help > Updates, or check the Updates listings in the Downloads section of the Adobe site.