How to view Microsoft Document Imaging (MDI) files on a Mac?
MDI files were a proprietary file format used by Microsoft Office 2003 and 2007 on Windows. Searching around online, some people have luck opening them with QuickTime (which is already on your Mac), or GraphicConverter. Not having access to these files, nor a direct Mac equivalent, without more information we would be unsure if this would work or not. Office 2008 and 2011 on the Mac won't open them.

The best way to view these (and free too, really), would be to have a Windows using friend to open them in Windows and export as another file type.