Try Catch Not working

How to I redirect the page if the Pdf file doesn t exist ,
I tried try ..catch but it s not redirecting  FirstPage.aspx


  Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load


        Dim fn As String = Request.QueryString("f")
        Try
            PdfBook.HRef = fn
        Catch ex As Exception
            Response.Redirect("FirstPage.aspx")
        End Try


    End Sub

generate error
Server Error in '/Issam May 2012' Application. The resource cannot be found. Go to the complete details ...