This is an open-source, XML-based format. Think of IDML as the "universal translator" for InDesign. It strips away version-specific metadata and saves the file’s core content—text, images, styles, and pages—into a readable code. Any version of InDesign from CS4 onward can open an IDML file. Furthermore, IDML is the standard format for importing into alternative layout software like QuarkXPress, Affinity Publisher, and Scribus.
Converting an Adobe InDesign (.INDD) file to the InDesign Markup Language (.IDML) format is essential for opening documents in older versions of InDesign or other layout software like Affinity Publisher. While InDesign itself is the primary tool for this, several online and third-party solutions exist for those without an active Creative Cloud subscription. Top Online INDD to IDML Converters indd to idml online converter
AConvert is a no-frills tool. It is not great for complex graphics, but if you just need to get the text out of a locked INDD file to paste into Word, it works instantly. This is an open-source, XML-based format