Code Block and File Download

So i create a lot of custom blocks via the HTML block and it works very good until i want to include a handout in my HTML code. 

 <a href="assets/filename.pdf" class="download-btn" download target="_blank">
                <svg viewBox="0 0 24 24">
                    <path d="M19 9h-4V3H9v6H5l7 7 7-7zM5 18v2h14v-2H5z"/>
                </svg>
                Merkblatt herunterladen (PDF)

The file is in the assets folder and it works when i click it on my desktop. When I use it in the Preview of Rise the button is unresponsive. Is this a known issue or am I doing something wrong?