iRule to fix F5 disconnect when opening PDF on published URL

i have an F5 Big-IP running 13.1.0.5. i have a Web URL published going to a back end Web Server. everything works fine, except when a user tries to open PDF content hosted on the Server the user gets disconnected from the Big-IP and from the application automatically. other than that the web application works fine. all other content is view-able.

How can i write an iRule to handle this type of scenario.

NB: if you access Web server direct (not through F5) you can view the PDF fine. so this must be an F5 issue.