public byte[] MergePdfs(IEnumerable<byte[]> inputFiles)
{
    MemoryStream outputStream = new MemoryStream();
    Document document = new Document();
    PdfWriter writer = PdfWriter.GetInstance(document, outputStream);
    document.Open();
    PdfContentByte content = writer.DirectContent;

    foreach(byte[] input in inputFiles)
    {
        PdfReader reader = new PdfReader(input);
        for (int i = 1; i <= reader.NumberOfPages; i++)
        {
            document.SetPageSize(reader.GetPageSizeWithRotation(i));
            document.NewPage();
            PdfImportedPage page = writer.GetImportedPage(reader, i);
            int rotation = reader.GetPageRotation(i);
            if (rotation == 90 || rotation == 270)
            {
                content.AddTemplate(page, 0, -1f, 1f, 0, 0,
                    reader.GetPageSizeWithRotation(i).Height);
            }
            else
            {
                content.AddTemplate(page, 1f, 0, 0, 1f, 0, 0);
            }
        }
    }
    document.Close();

    return outputStream.ToArray();
}