Print Preview Error

[istiyaque]

hi,

I am intermediate in programming and i am getting error on print preview command in my program.
Actually i had made a website using vb.net code. and i have a DailyUpdateReport.aspx page. in that i putted one image button and on image button i putted the code below..

Private Sub ImagebuttonPP_Click(ByVal sender As System.Object, ByVal e As System.Web.UI.ImageClickEventArgs) Handles ImagebuttonPP.Click
        Try
            Dim oStringWriter As New System.IO.StringWriter
            Dim oHtmlTextWriter As New System.Web.UI.HtmlTextWriter(oStringWriter)
            'Dim oHtmlTextWriter As New System.Xml.XmlTextWriter(oStringWriter)
            Response.Clear()
            Response.Buffer = True
            Response.ContentType = "application/pdf"
            Response.Charset = ""
            Me.EnableViewState = False
            DG1.RenderControl(oHtmlTextWriter)
            Response.Write(oStringWriter.ToString())
            Response.End()
        Catch ex As Exception
            lblErr.Visible = True
            lblErr.Text = ex.Message
        End Try
    End Sub

before few months its working fine and opening the that aspx page as on print preview in pdf. but now its giving error
' File does not begin with %pdf- '
I am using a Adobe Reader 9, version 9.3.2
and Internet Explorer 8
i don't know what is the problem. and how to resolve the problem.
can anyone knows the solution than please help out.
if u r not understand anything on problem then let me know to explain.
its very urgent.

Thanks
Ishtiyaque.