Merge to pdf files from their stream objects into a MemoryStream using Aspose.Pdf.

MergePdf.cs

        static MemoryStream MergePdfDocuments(Stream pdf1, Stream pdf2)
        {
            MemoryStream result = null;

            Aspose.Pdf.Document pdf1Document = new Aspose.Pdf.Document(pdf1);
            Aspose.Pdf.Document pdf2Document = new Aspose.Pdf.Document(pdf2);

            Aspose.Pdf.Document pdfResult = new Aspose.Pdf.Document();

            pdfResult.Pages.Add(pdf1Document.Pages);
            pdfResult.Pages.Add(pdf2Document.Pages);

            result = new MemoryStream();

            pdfResult.Save(result);

            result.Position = 0;

            pdf1Document.Dispose();
            pdf2Document.Dispose();
            pdfResult.Dispose();

            return result;
        }