public static readonly string InputFileMultipageDocument = @"..\..\Lorem.pdf";
public static readonly string InputFileWatermark = @"..\..\Watermark.pdf";

private void RadButton_Click(object sender, RoutedEventArgs e)
{
    PrependAndAppendPageContent("Result.pdf");
}
private static void PrependAndAppendPageContent(string resultFileName)
{
    string initialDocument = InputFileMultipageDocument;
    string resultDocument = Path.Combine(@"..\..\", resultFileName);

    using (PdfStreamWriter fileWriter = new PdfStreamWriter(File.OpenWrite(resultDocument)))
    {
        using (PdfFileSource fileSource = new PdfFileSource(File.OpenRead(initialDocument)))
        {
            RadFixedPage backgroundContentOwner = GenerateWatermark(InputFileWatermark);

            foreach (PdfPageSource pageSource in fileSource.Pages)
            {
                using (PdfPageStreamWriter pageWriter = fileWriter.BeginPage(pageSource.Size, pageSource.Rotation))
                {
                    using (pageWriter.SaveContentPosition())
                    {
                        double xCenteringTranslation = (pageSource.Size.Width - backgroundContentOwner.Size.Width) / 2;
                        double yCenteringTranslation = (pageSource.Size.Height - backgroundContentOwner.Size.Height) / 2;
                        pageWriter.ContentPosition.Translate(xCenteringTranslation, yCenteringTranslation);
                        pageWriter.WriteContent(backgroundContentOwner);
                    }

                    pageWriter.WriteContent(pageSource);

                }
            }
        }
    }
}

private static RadFixedPage GenerateWatermark(string watermark)
{
    using (Stream imageStream = File.OpenRead(watermark))
    {
        PdfFormatProvider provider = new PdfFormatProvider();
        var docuemnt = provider.Import(imageStream);

        return docuemnt.Pages.First();
    }
}