Converting to IDML is often used for "down-saving" to older InDesign versions, but some newer features (like specific typography settings or interactive elements) may be lost in the process If you'd like, let me know: How many files you need to convert? If you have access to any design software (like InDesign, Affinity, or QuarkXPress)? InDesign version the original file was created in?
Converting INDD to IDML can be useful in various scenarios: online converter indd to idml full
While primarily for PDFs, Markzware offers high-end conversion technology that handles InDesign structures better than almost any other provider. Converting to IDML is often used for "down-saving"