Document

What is a .indd file?

A .indd file is an Adobe InDesign Document (version Generic). This is a document format, so it holds text and layout meant to be read. Confusingly, this extension has been used by 22 different formats over the years — they're all listed below.

This extension has been used by more than one format

22 different formats have claimed .indd over the years, so two files with the same extension aren't necessarily the same kind of thing.

How to open a .indd file

Try LibreOffice (free, Windows/Mac/Linux) — it opens an enormous range of current and historic document formats.
Microsoft Word and Google Docs handle most mainstream document types.
If nothing opens it, open it in a text editor: many older document formats have readable text inside.

The registry records this extension against Adobe InDesign Document, Adobe InDesign Document, Adobe InDesign Document and others, so software from that family is the most likely match.

How to convert a .indd file

Once it opens in LibreOffice or Word, Save As .docx or .pdf to get it into a format you can use anywhere.

MIME type

When a .indd file travels over the web, servers label it with this MIME type:

application/octet-stream

What's a MIME type?

The technical record

For archivists, developers and the merely curious: these are the PRONOM registry entries for .indd.