For mission-critical files, use Adobe InDesign itself (File → Save As → IDML). For quick, simple INDD files, the online tools below are verified as functional but limited.
If you have access to InDesign, the most verified method isn't an online tool at all—it’s the indd to idml converter online verified
Future developments and enhancements to the converter may include: For mission-critical files, use Adobe InDesign itself (File