Protected Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs) Handles Button1.Click
        Dim fileData As Byte()
        Using connection As New SqlConnection(ConfigurationManager.ConnectionStrings("xxx").ConnectionString)
            'some code...
            connection.Open()
            fileData = DirectCast(command.ExecuteScalar(), Byte())
            connection.Close()
        End Using

        Dim mypdf As New System.IO.MemoryStream(fileData)

        'Dim document As New PdfDocument(mypdf)
        Dim document As PdfDocument = New PdfDocument()
        document = PdfReader.Open(mypdf)

        Dim page As PdfPage = document.Pages(0)
        Dim gfx As XGraphics = XGraphics.FromPdfPage(page, XGraphicsPdfPageOptions.Prepend)

        Dim MyWaterM As String = "WaterMark"
        ' Get the size (in point) of the text
        Dim options As XPdfFontOptions = New XPdfFontOptions(PdfFontEncoding.Unicode, PdfFontEmbedding.Always)
        Dim font As XFont = New XFont("Calibri", 12, XFontStyle.Regular, options)
        Dim size As XSize = gfx.MeasureString(MyWaterM, font)

        ' Define a rotation transformation at the center of the page
        gfx.TranslateTransform(page.Width.Point / 2, page.Height.Point / 2)
        gfx.RotateTransform(-Math.Atan(page.Height.Point / page.Width.Point) * 180 / Math.PI)
        gfx.TranslateTransform(-page.Width.Point / 2, -page.Height.Point / 2)

        ' Create a string format
        Dim format As New XStringFormat()
        format.Alignment = XStringAlignment.Near
        format.LineAlignment = XLineAlignment.Near

        ' Create a dimmed red brush
        Dim brush As XBrush = New XSolidBrush(XColor.FromArgb(128, 255, 0, 0))

        ' Draw the string
        gfx.DrawString(MyWaterM, font, brush, New XPoint((page.Width.Point - size.Width) / 2, (page.Height.Point - size.Height) / 2), format)

        Dim stream As New MemoryStream()
        document.Save(stream, False)
        Dim data As Byte() = stream.ToArray()

        Response.ContentType = "application/pdf"
        Response.AddHeader("content-length", fileData.Length.ToString())
        Response.BinaryWrite(data)
        Response.End()

    End Sub

gfx.DrawString(MyWaterM, font, brush, New XPoint((page.Width.Point - size.Width) / 2, (page.Height.Point - size.Height) / 2), format)

Response.AddHeader("content-length", fileData.Length.ToString())

Response.AddHeader("content-length", data.Length.ToString())