Hm... already found the answer myself ... not possible with the wkhtmktopdf engine
Found this tool https://www.pdflabs.com/tools/pdftk-server/ ... maybe it's possible to integrate it

HI there
Of course you can .Here is the code:

using RasterEdge.Imaging.Basic;
using RasterEdge.Imaging.Basic.Core;
using RasterEdge.Imaging.Basic.Codec;
using RasterEdge.Imaging.PDF;


namespace RE__Test
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        public static string FolderName = "c:/";

        private void button1_Click(object sender, EventArgs e)
        {
            string fileName = FolderName + "SampleNew.pdf";

            PDFDocument pdf = new create PDF Document(5);

            REFile.SaveDocumentFile(pdf, fileName, new PDFEncoder());//save new pdf

        }