The MDI (Microsoft Document Imaging) file extension was originally created by Microsoft as one of the components of its office suite.
The file type was used by Microsoft to save documents that were scanned into the system.
Once scanned, an optical character recognition software, called OCR, could be used to scan the graphics in the document, detect the characters, and convert them into words, resulting in a common Word file.
Typically an MDI document can only be created through a Microsoft product specially designed for this function.
Microsoft Office Document Imaging, MODI, is the most common software for this task.
Although the MODI format was a standard within Office XP for several years, it is currently discontinued, so if you have an old document created with MODI and it has the MDI extension, you won't be able to open it unless you have the specific old version of Office XP.
You will need to convert the MDI file to one that your operating system can handle.
For more general information about files, file extensions and registry you can read one of the following articles:
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