MDI: Multiple Document Interface is when windows reside under a single parent application window.

SDI: Single Document Interface is when all document windows are separate from each other.

MDI is not applicable on the Mac OS. On Mac OS X, an MDI mode does not "save screen real estate" by eliminating redundant menu bars, because the Mac OS X graphical user interface is application-centric instead of window-centric. As opposed to Windows, all windows belonging to an application share the same menu. Therefore, when discussing MDI and SDI it is a Windows specific conversation.

Some of our customers have noticed a change in the viewing modes of Acrobat and Adobe Reader on Windows over the last two versions. Acrobat and Adobe Reader had long displayed documents in MDI mode. In Acrobat and Adobe Reader 7 we introduced the ability to view documents in SDI mode. This was not the default view mode, but a user could enable it in the preferences. In Acrobat and Adobe Reader 8 we made the default mode SDI and still allowed users to choose MDI in the preferences.

Acrobat and Adobe Reader said good-bye to MDI mode in version 9. Probably not noticed by most of you, but some of you have noticed and asked why we did this. The Acrobat Product Management team wanted to share our reasoning with you for this change.

There were several contributing factors that led to this decision.

1. Feature Parity with Macintosh was desired. As stated before, the Mac OS does not have this option. While I will be the first to admit that feature parity is not 100% between Windows and Macintosh, it is a goal that we aim for.
2. In version 8, we made SDI the default in the viewing mode. Making SDI default, but still providing MDI in version 8 was done to start the deprecation of MDI.
3. Microsoft advised that to work as good as possible on Vista, applications should avoid MDI.
4. Acrobat and Adobe Reader’s new UI modes would not work with MDI. Form editing mode, portfolio mode, and portfolio preview mode all wanted a complete refresh of the UI. MDI mode always left a bit of the UI skin under the care of an MDI main frame, so there would have had been no way for those UI modes to re-skin that part of the UI if MDI mode was left in place.
5. A cost compelling reason was that MDI and SDI mode essentially became another view mode in which all work flows had to be tested. This increases the cost of testing the product and the cost of fixing bugs. Often a fix to a bug in one view mode would cause adverse reactions in the other view mode. The decision to support only one view mode on Windows was made to simplify this. Furthermore, more time spent in this area could mean less time spent developing and fixing bugs in other areas.

MDI mode seems to have been loved for the aspect of keeping all documents opened by a single application contained tightly together. However, in a world where larger screen resolutions, multiple monitors and the need to see multiple documents at one time has increased, SDI mode offered higher benefits. Couple all this with the requirements coming from Windows Vista and the decision was made to drop MDI mode.

Going forward the Acrobat Product Management team has started exploring alternative ways to accomplish some of what is so loved about MDI mode. While we can’t comment publicly on that at this time, please know that we have heard you. I hope the reasoning I provided helps you understand the choices we made a little bit better as well. Please let us know your thoughts about the removal of MDI support. Let us know what you feel the benefits of MDI are and how it helps you get your work done. We would love to hear from you.

Jeff Moran

Acrobat Product Management

source:  http://en.wikipedia.org/wiki/Multiple_document_interface