I upgraded the default version of Internet Explorer to IE 9 but it brings me an issue while opening PDF files. (Error message is attached as follow). I have tried opening it via Adobe Reader 10 and 9 but nothing works fine. Before upgrading the IE to version 9, I did not encounter any kind of issue related to opening PDF files.
The error message below appeared on the screen:
“The Adobe Acrobat/Reader that is running can not be used to view PDF files in a Web Browser.
Adobe Acrobat/Reader version 8 or 9 is required. Please exit and try again.”

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Try this:
Reinstall the Adobe/ Acrobat
Click - Start -> control panel > programs and features (Windows 7 or Vista) or add or remove programs (XP) > select Adobe version > choose unistall/remove programs
Repeat the unistall to all adobe versions.
Restart your PC and then reinstall
Note: Before you reinstall the program, make sure you unistall adobe versions. You may check as well C://Program files/adobe/ and delete all the files for adobe (This would help to have a better results after the reinstallation)
Also try this:
Open the Acrobat
Select the "help" on the menu then select "repair adobe reader installation"