In an article, I inserted this code:
<p style="text-align: center;"><a href="https://www.joomlacontenteditor.net/images/Articoli/0001_0050/0005/Studio_Il_loto_20231215_Cookie.pdf">
<button class="btn"> <strong>Leggi l'informativa estesa</strong><br><small>sui cookie</small></button>
</a></p>
And everything works well: when I click the button, the PDF document opens.
If I add the modal class (by Regolar Labs) to the link:
<p style="text-align: center;"><a href="https://www.joomlacontenteditor.net/images/Articoli/0001_0050/0005/Studio_Il_loto_20231215_Cookie.pdf" class="modal">
<button class="btn"> <strong>Leggi l'informativa estesa</strong><br><small>sui cookie</small></button>
</a></p>
in the frontend there is no problem at all, but in the backend editor (JCE) the button disappears. It is no longer visible, but it is still present (and working) in the Code tab.
You can test the page here: www.studioilloto.it/info-legali/cookie
The image shows the backend editor appearance: only a second button (with no modal class) is visible.