The Microsoft Document Imaging was originally released with Microsoft Office XP and was included in both Office 2003 and Office 2007 - it has since been discontinued. The format was developed to allow users to scan documents, then OCR (optical character recognition) those documents, view them and if needed annotate them. MDI or MODI as it is otherwise known is an extension of the TIFF file format.
MDI or MODI is a proprietary format which stores scanned documents and can include annotations and/or metadata. MDI files are limited in their nature in that they can only be produced or read by MODI applications or applications that invoke MODI modules. MDI files are an extension of the TIFF file format, however their are a number of differences - with MDI files, three image compression formats are used, a number of individual tag values and a magic number of 0x5045 instead of 0x4D4D.
PDF is a file format developed by Adobe Systems for representing documents in a manner that is separate from the original operating system, application or hardware from where it was originally created. A PDF file can be any length, contain any number of fonts and images and is designed to enable the creation and transfer of printer-ready output.
Each PDF file encapsulates a complete description of a 2D document (and, with the advent of Acrobat 3D, embedded 3D documents) that includes the text, fonts, images and 2D vector graphics that compose the document. They do not encode information that is specific to the application software, hardware, or operating system used to create or view the document.