The problem with this format is that it's not very handy - you can't even open it with many applications, never mind edit it.
MDI2PDF is a tool that allows you to convert MDi files to PDF or JPG format, which is much more useful if you want to later send or edit the image.
The basic version of MDI2PDF includes a couple additional options, like mirroring or rotating the image, adjusting the screen, or converting all files within a given folder. However, since its main purpose is to convert your MDI files into a more manageable format, if you need to do some further editing, it would be easier to just use a conventional image editor.
Although the basic version of MDI2PDF is free, you can unlock more features with a paid version.