fancybox plugin tries to display PDFs

Hi
I use preview images for a PDF download.
The fancybox plugin is trying to display the PDF like an image.
I would expect that the plgin does not touch links to a PDF. It looks like this

<div class="itemP">
<a href="/fmdata/presse/data2020/5_Postkarten%20und%20Plakat,%20etc/din-lang-2020-NL.pdf" target="_blank" class="fancyboxforwp" data-fancybox="gallery">
<img src="/fmdata/presse/data2020/5_Postkarten%20und%20Plakat,%20etc/din-lang-2020-NL-t.jpg" alt=""
><br>din-lang-2020-NL<br> (PDF  MB) <br><br>
</a>
</div>

any suggestion to solve this ?
thanks
Alex