Use this guide to navigate the murky waters of InDesign file formats, and wise-up on the difference between INDD and IDML, and the right way.
INDD is a file extension for a page layout file used by Adobe InDesign, can also be opened using Adobe InCopy. INDD stands for INDesign Document. Contains.
Adobe InDesign is a desktop publishing software application produced by Adobe Systems. Filename extension. indd. Internet media type, application/x- Stable release : CC 2017 / November 2, 2016.