MDI stands for Microsoft Document Imaging format. This TIFF-based file format is designed to store images by page layout using Office Document Imaging, which can save files as MDIs and TIFFs. TIFF files are widely compatible, but MDI files only open with Office Document Imaging, which can be a problem when you have to share MDI files. The best solution is to save them as TIFFs to begin with, but that's not much help if you're the one trying to open MDI files without Office. MDI Viewer by HAN Software opens and displays MDI and TIFF images without having Office, Adobe Acrobat, or any other big-bucks, big-box wares. It also displays most common image formats, such as JPEG and PNG, and supports batch conversion of MDI and image files to PDF.
MDI Viewer's interface is efficient but not especially attractive, with some unappealing color choices. That's a matter of opinion, of course, but what isn't is the way the program's interface opened maximized, with the size controls underneath our right-hand Windows toolbar. We had to unlock the toolbar and drag it to the bottom of our desktop to uncover MDI Viewer's controls and minimize the window. MDI Viewer displays thumbnails in its left-hand navigation panel. It does indeed open MDI and TIFF files as well as a variety of other image formats, but it wouldn't open any of our PDFs, claiming the files were missing or corrupted (they weren't, as we quickly verified). Stranger still, we found no evidence of any conversion capability, or even the ability to save files; just open them. A button labeled More Functions took us to the developer's Web site and additional software offers. The Help file is for MDI Converter, which offers more features but isn't free.
If you need to view MDI files without the appropriate Microsoft programs, your choices are limited. MDI Viewer will let you see the contents of MDI files, which might be all you need, and you can always print the screen or take a screenshot if you need to save data. That's cumbersome, but it beats the alternative, if there is one.