If you need to perform this "magic trick" yourself, here is the standard process: the file in your newer version of InDesign. InDesign Markup (IDML) from the "Save as type" or "Format" menu. the file to your desired location. file using the older version of InDesign. the file immediately as a standard to lock in the format. ⚠️ Important Limitations
The standard way to "downsave" a file is through the export function in the newer version of InDesign. in the more recent version of InDesign. how to convert indesign file to older version
If you are a Creative Cloud subscriber, Adobe offers a "Simple Saveback" service. If you need to perform this "magic trick"
is a standalone application (no Adobe required) that opens any version of InDesign file and converts it to: how to convert indesign file to older version