Lightbox Pdf not working – shows up black multiple solutios

can you try this:
place that snippet inside your child-theme functions.php:

function lightbox_with_mixed_content_type() { 
?>
<script type="text/javascript">
window.addEventListener("DOMContentLoaded", function () { 
  (function($){
    $('.mixed-contenttype a').each(function(){
      var anchorLink = $(this).attr('href');
      if (/\.(jpg|png|gif|webp|jpeg)$/.test(anchorLink)) {
        //console.log(anchorLink + " is an image link");
      } else {
        $(this).attr('rel', 'lightbox').addClass('mfp-iframe');
      }
    }); 
  })(jQuery);
});
</script>
<?php 
}
add_action('wp_footer', 'lightbox_with_mixed_content_type', 999);

on your page place a button and give that custom class to the elment (or to one parent element) : mixed-contenttype
now link to your pdf.
See: https://webers-testseite.de/pdf-in-lightbox/
But: Not all browsers will display this, especially mobile browser variants.

INFO: due to GDPR ( DSGVO in Germany ) i can not show the working script for video content – it is hard to block this – playing videos from a masonry or from an image linked to a video.