Every newer version of InDesign creates new version of IDML format, which the previous version cannot open. This means that users look for ways how to export their modern files to older formats. Adobe offers conversion service for Creative Cloud subscribers. You can also find some commercial 3rd party services that offer InDesign conversion for free.
INX format is interchange format that is often used to import newer InDesign files to older version, for example INX file exported in CS4 can be imported in CS3.Open IDML file Open INX file
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