Document

What is a .indt file?

A .indt file is an Adobe InDesign Document (version CC 2014). This is a document format, so it holds text and layout meant to be read. Confusingly, this extension has been used by 21 different formats over the years — they're all listed below.

This extension has been used by more than one format

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

How to open a .indt 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 .indt 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 .indt 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 .indt.