How do i modify the application so that it download the file

Question

I am working on old asp.net web form application which opens the file on button click.

I need to modify the application so that it download the file instead of opening the file.

How do i do it?

Code behind :

on page load event :

btnAdvertising.OnClientClick = "javascript:window.open('" & My.Settings.FilePath & "');"

btnAdvertising.DataBind()

In my config file I have passed physical filepath:

<setting name="MAG_URL" serializeAs="String">

<value>file://NetworkDrvie/SHARED/AllPdf/Advertising.pdf</value>

</setting>