In some scenarios, you may be unable to open a PDF document in Windows Internet Explorer 7 or in Internet Explorer 6. In some scenarios, a PDF document may not appear correctly in Internet Explorer.
This problem occurs because of the method that is used to determine the MIME type of downloaded content. This problem occurs even if the content MIME type is declared as "application/pdf."
947864 MS08-024: Cumulative Security Update for Internet Explorer
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.