While not "free," this is a valuable mention for those needing to convert files without installing any software.
Open InDesign documents in previous versions - Adobe Help Center
The .indd format is also heavily , containing cached preview data, image thumbnails, and efficiency structures that make working with complex layouts faster within the same version. This performance optimization comes at the cost of cross-version compatibility.
Before you start converting, please be aware of these caveats: how to convert indesign file to older version free
Converting an file to a previous version (often called "downsaving") is a common necessity when collaborating with others who haven't updated their software. While Adobe’s native method is the most reliable, there are a few ways to handle this without a paid third-party plugin. The Standard Method: IDML Files
If the newer file utilized a third-party plugin for barcoding, data merge, or special pagination, the older version will throw a warning. You will need to install the older version of that specific plugin to fix it.
Find out how to with clients who do not own Adobe software. Share public link While not "free," this is a valuable mention
When you're sharing files with someone using an older version, simply sending an IDML file may not be enough—they'll also need all the linked images and fonts. InDesign's feature can bundle everything together, including an IDML version for compatibility.
The fastest and safest method is to ask the original creator to "Save As" an IDML file and send it to you. This ensures the file is converted properly within the software, as described in the Adobe Community . 2. Use Adobe Creative Cloud Desktop
While convenient, online converters have significant limitations: Before you start converting, please be aware of
What to Expect After Conversion (Troubleshooting Layout Changes)
Newer versions of InDesign often have improved composition engines. Opening a file in an older version might cause text to wrap differently, potentially creating overset text.
Many free online tools have file size restrictions (typically 10-50MB). Large magazine or book layouts may exceed these limits.
“Hey! I’m running an older version of InDesign. Can you please open the file, go to File > Export, choose ‘InDesign Markup (IDML),’ and send me that file instead?”