IDML stands for InDesign Markup Language. Introduced by Adobe in InDesign CS4, IDML is an XML-based format that describes the entire layout of an InDesign document in a universal language. Because it uses XML, it is completely backward-compatible. This means an IDML file generated from the absolute newest version of InDesign can be opened by older versions of the software without any compatibility warnings. Why Use an Online INDD to IDML Converter?