After installing Adobe Reader 9 I get this error: The Adobe Acrobat/Reader that is running can not be used to view PDF files in a Web Browser. Please exit Adobe Acrobat/Reader and exit your Web Browser and try again.
In Firefox click on Tools->Options and then on the Applications tab
Set the action for the Content Type: pdf File to Use Adobe Reader 9.x and the problem will be solved
Image Credits: Photo by Isis França on Unsplash.