How to Find the Missing Plug-ins for Adobe Readerby Ray Padgett
Adobe Reader offers a browser plug-in to let you view PDF files in Firefox, Internet Explorer and Google Chrome. Although useful, it contains an error that makes it occasionally disappear from browsers -- Firefox in particular -- when you try to enable it. To display the missing plug-in, you have to reset it in the Adobe Reader preferences.
Quit your Web browser.
Open "Adobe Reader."
Select "Preferences" from the "Edit" menu, and click the "Internet" tab.
Unselect "Display PDF in Browser" and click "OK." If it is already not selected, you can skip this step.
Return to the "Internet" tab, but this time select "Display PDF in Browser" and click "OK."
Open your Web browser again. Toggling the Reader preferences will have fixed the issue.