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What is a .apr file?

A .apr file is an ESRI Arc/View Project (version 2.x). This one is a specialist format without an obvious everyday home. Confusingly, this extension has been used by 3 different formats over the years — they're all listed below.

This extension has been used by more than one format

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

How to open a .apr file

The program that created it is by far the most reliable way to open it.
On Windows, use a hex viewer or the free TrID tool to identify what the file really is.
On Mac or Linux, running `file yourfile` in the terminal identifies most formats instantly.

The registry records this extension against ESRI Arc/View Project, Lotus Approach View File, ESRI Arc/View Project, so software from that family is the most likely match.

How to convert a .apr file

Identify the file first, then look for an export or Save As option in whatever program can read it.

MIME type

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

application/vnd.lotus-approach

What's a MIME type?

The technical record

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