Response.Clear()
Response.ContentType = result.MimeType 'this is always "application/pdf"
Response.Cache.SetCacheability(HttpCacheability.Private)
Response.Expires = -1
Response.Buffer = True
Response.AddHeader("Content-Disposition", String.Format("{0};FileName={1}", "attachment", fileName))
Response.BinaryWrite(result.DocumentBytes)
Response.End()

Protected Sub ButtonPDF_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnPDF.Click

    DownloadReportPDF(Me.RadGrid1.SelectedValue.ToString())

    Dim strMessage As String = "alert('Printed PDF Sheet.');"
    ScriptManager.RegisterStartupScript(Me, Me.GetType, "MyScript", strMessage, True)

End Sub

Protected Sub DownloadReportPDF(ByVal releaseMasterId As String)
    'Service call to generate report source
    Dim service As New TelerikReportLibrary.ReportServices.PPSReportService
    Dim source As Telerik.Reporting.TypeReportSource = service.GetReportSource(releaseMasterId)

    'Render PDF and download
    Dim reportProcessor As New ReportProcessor()
    Dim result As RenderingResult = reportProcessor.RenderReport("PDF", source, Nothing)

    Dim fileName As String = result.DocumentName + "_" + releaseMasterId + "." + result.Extension

    Response.Clear()
    Response.ContentType = result.MimeType 'this is always "application/pdf"
    Response.Cache.SetCacheability(HttpCacheability.Private)
    Response.Expires = -1
    Response.Buffer = True
    Response.AddHeader("Content-Disposition", String.Format("{0};FileName={1}", "attachment", fileName))
    Response.BinaryWrite(result.DocumentBytes)
    Response.End()

End Sub

<asp:linkButton ID="btnPDF" CssClass="btn btn-default" runat="server" Width="115px">
        <i class="fa fa-file-text" title="Edit"></i> PDF
    </asp:linkButton>