IE hide the Menu Items when PDF is loaded on Webform

I've a menu item when where user click on menu item button(Claim) it open the PDF below the menu item. Image attached shows how it's done Check Image

Issue is when i loaded the same webform in Internet Explorer it's Open the PDF below the code but my menu item list goes behind the PDF.Image is attached to explain more Error Image

To explain more I'm attaching my code which help to understand how PDF is loading on web form.

<iframe id="myframe" width="100%" height="600px" runat="server" frameborder="1" ></iframe>

I've used the iframe to load the PDF.

string fileName = "IGI-Travel-Sure-Claim-Procedure.pdf";
    this.myframe.Attributes["src"] = "../uploads/pdf/" + fileName;

Above code is when form loaded then it load the PDF on web form.

My issue the that the whenever i open my webfrom in internet explorer it hide my menu item list behind the iframe PDF viewer. Meanwhile same webform with same code and iframe works fine in all browser like Chrome,FireFox,Opera and Edge too but on in Internet Explorer 11. The application is developed in framework 2.0