From Acrobat Reader 5 - select Edit/Preferences/Options. Disable the entry: "Allow fast web view". Save the changes and reopen the web browser.  Now the PDF is displayed.

From Acrobat Reader 6 - Open the Adobe Acrobat Reader, click Edit, click Preferences, select "Internet" in the table on the left, uncheck "Allow Fast Web View.",  and click OK to save the changes.