Try something like this:

C#

protected void Button4_Click(object sender, EventArgs e)
{
        string path = @"D:\Data\Monthly Report\March.pdf ";
        Page.ClientScript.RegisterStartupScript(this.GetType(), "Pop up", "window.open('" + path + "','','');", true);
}

VB.NET

 Protected Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs)
    Dim path As String = "D:\Data\Monthly Report\March.pdf "
    Page.ClientScript.RegisterStartupScript(Me.[GetType](), "Pop up", "window.open('" & path & "','','');", True)
    
End Sub

You may also look here:

http://dotnetguts.blogspot.com/2006/09/popup-pdf-file-in-new-window-from.html