Files that contain the .mdi file extension are image files that are created by the Microsoft Office Document Imaging program. The MDI files created by this application are tag-based graphics that are commonly used when you print to the Microsoft Office Document Image Writer.
Files with the .mdi extension are very much like TIFF files, but the MDI file format is proprietary to the Microsoft application. Files with the .mdi extension can store text that is recognized by optical character recognition (allowing you to edit the text in the files with compatible software) as well as graphics and file layouts. These files can later be printed in hard copy.
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If you have additional information about the MDI file format or software that uses files with the MDI suffix, please do get in touch - we would love hearing from you.