// Open the template file as image
XImage image = XImage.FromFile(pdfTemplatePath);

// Open an existing document for editing and loop through its pages.
var document = PdfReader.Open(filename);
// Set version to PDF 1.4 (Acrobat 5) because we use transparency.
            if (document.Version < 14)
                document.Version = 14;

            for (var idx = 0; idx < document.Pages.Count; idx++)
            {
                var page = document.Pages[idx];

                // Get an XGraphics object for drawing beneath the existing content.
                var gfx = XGraphics.FromPdfPage(page, XGraphicsPdfPageOptions.Prepend);
             

                gfx.DrawImage(image);

            }
            // Save the document...
            document.Save(filename);

byte[] bytes = System.Convert.FromBase64String(fileBase64);
MemoryStream ms = new MemoryStream(bytes);
XImage image= XImage.FromStream(ms);

if (uri.StartsWith("base64:"))
{
    string base64 = uri.Substring("base64:".Length);
    byte[] bytes = Convert.FromBase64String(base64);
    {
        Stream stream = new MemoryStream(bytes);
        XImage image = XImage.FromStream(stream);
        return image;
    }
}
return XImage.FromFile(uri);

// simulate a base64 string in
Byte[] bytes = File.ReadAllBytes(path);

//byte[] bytes = System.Convert.FromBase64String(fileBase64);
MemoryStream ms = new MemoryStream(bytes);
XImage image= XImage.FromStream(ms);

byte[] bytes = Convert.FromBase64String(base64);
using (Stream stream = new MemoryStream(bytes))
{
      XImage image = XImage.FromStream(stream);
      return image;
}

protected void Page_Load(object sender, EventArgs e)
        {
                // simulate an imput a real PDF file
                Byte[] bytes = File.ReadAllBytes(@"D:\file5mb.pdf");
                MemoryStream stream = new MemoryStream(bytes);
                XImage image = XImage.FromStream(stream);     
        }

[ArgumentException: Invalid parameter.]
   System.Drawing.Image.FromStream(Stream stream, Boolean useEmbeddedColorManagement, Boolean validateImageData) +1402079
   PdfSharp.Drawing.XImage..ctor(Stream stream) +139
   PdfSharp.Drawing.XImage.FromStream(Stream stream) +43
   System.Web.UI.Control.OnLoad(EventArgs e) +109
   System.Web.UI.Control.LoadRecursive() +68
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +4498

XPdfForm xpfTemplatePdf = XPdfForm.FromStream(xpfTemplateStream);

XPdfForm xpfMultiPagePdf = XPdfForm.FromStream(xpfMultiPageStream);

// Create the Template XPdf from a MemoryStream
XPdfForm xpfTemplatePdf = XPdfForm.FromStream(templateMS);

// Create a Document from a MemoryStream
var document = PdfReader.Open(docMS);

for (var idx = 0; idx < document.Pages.Count; idx++)
{
    var page = document.Pages[idx];

    // Get an XGraphics object for drawing beneath the existing content.
    var gfx = XGraphics.FromPdfPage(page, XGraphicsPdfPageOptions.Prepend);

    gfx.DrawImage(xpfTemplatePdf, 0, 0, page.Width, page.Height);

}
// Save the document...
document.Save(@"D:\fattura.pdf");

public PDFConcatenate(PackageToJoin pdfPackage)
        {
            MemoryStream outStream = new MemoryStream();

            CreateBaseDocument(pdfPackage).Save(outStream, false);
            ReturnFile = outStream.ToArray();
            DocName = pdfPackage.Name + ".pdf";
        }

        private PdfDocument CreateBaseDocument(PackageToJoin package)
        {
            OutputDocument = new PdfDocument();
            CreateHeaderAndFooter(package);
            foreach (var chunk in package.Chunks)
            {
                InputDocument = PdfReader.Open(new MemoryStream(chunk.Data), PdfDocumentOpenMode.Import);
               
                int count = InputDocument.PageCount;

                for (int idx = 0; idx < count; idx++)
                {
                    PdfPage page = InputDocument.Pages[idx];               
                    OutputDocument.AddPage(page);
                    AddHeaderFooter(OutputDocument.Pages[OutputDocument.PageCount-1], chunk.UseHeaderFooter);                 
                }
            }
            return OutputDocument;
        }

        private void AddHeaderFooter(PdfPage page, HeaderAndFooter headersAndFooters)
        {
            XGraphics gfx = XGraphics.FromPdfPage(page);

            if (headersAndFooters.ApplyHeader)
            {
                XRect adjustedHeaderRect = new XRect(5, 5, HeaderRect.Width,
                    HeaderRect.Height);
                gfx.DrawImage(Header, adjustedHeaderRect);
            }

            if (headersAndFooters.ApplyFooter)
            {
                XRect adjustedFooterRect = new XRect(5, page.Height - FooterRect.Height - 5, FooterRect.Width,
                    FooterRect.Height);
                gfx.DrawImage(Footer, adjustedFooterRect);
            }
            gfx.Dispose();
        }

        private void CreateHeaderAndFooter(PackageToJoin package)
        {
            //assign header form
            var rawHeader = PdfReader.Open(new MemoryStream(package.HeaderData), PdfDocumentOpenMode.Import);
            PdfPage page = rawHeader.Pages[0];
            HeaderRect = new XRect(0,0, page.Width, page.Height);
            Header = new XForm(OutputDocument, page.Width, page.Height);
            XGraphics headGraphics = XGraphics.FromForm(Header);
            headGraphics.DrawImage(XPdfForm.FromStream(new MemoryStream(package.HeaderData)), HeaderRect);
            headGraphics.Dispose();
           
            //assign footer form
            var rawFooter = PdfReader.Open(new MemoryStream(package.FooterData), PdfDocumentOpenMode.Import);
            page = rawFooter.Pages[0];
            FooterRect = new XRect(0,0, page.Width, page.Height);
            Footer = new XForm(OutputDocument, page.Width, page.Height);
            XGraphics footGraphics = XGraphics.FromForm(Footer);
            footGraphics.DrawImage(XPdfForm.FromStream(new MemoryStream(package.FooterData)), FooterRect);
            footGraphics.Dispose();
        }