Change button to an anchor a, and add download attribute to it. Look for this demo: https://davidwalsh.name/download-attribute
<a class="btn btn-modal" href="file.pdf" download="file.pdf" data-featherlight="iframe">
<img class="modal-button-icon" src="icon.svg" alt="Grundriss">
</a>
UPDATE: I misunderstood what you want. You can simply embed a PDF file in Bootstrap modal and show modal without Featherlight (there is a conflict between Bootstrap and Featherlight)
IMPORTANT!!!
PDF is not displayed in this code preview, please code the code from below to a HTML file and test it.
<!DOCTYPE html>
<html>
<head>
<title></title>
<link rel="stylesheet" type="text/css" href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.5.0/css/bootstrap.min.css">
</head>
<body>
<!-- Button trigger modal -->
<button type="button" class="btn btn-primary" data-toggle="modal" data-target="#exampleModal">
Launch demo modal
</button>
<!-- Modal -->
<div class="modal fade" id="exampleModal" tabindex="-1" role="dialog" aria-labelledby="exampleModalLabel" aria-hidden="true">
<div class="modal-dialog" role="document">
<div class="modal-content">
<div class="modal-header">
<h5 class="modal-title" id="exampleModalLabel">Modal title</h5>
<button type="button" class="close" data-dismiss="modal" aria-label="Close">
<span aria-hidden="true">×</span>
</button>
</div>
<div class="modal-body">
<object data="https://www.soundczech.cz/temp/lorem-ipsum.pdf" type="application/pdf" style="width: 100%; height: 600px;">
<embed src="https://www.soundczech.cz/temp/lorem-ipsum.pdf" type="application/pdf">
<p>This browser does not support PDFs. Please download the PDF to view it: <a href="https://www.soundczech.cz/temp/lorem-ipsum.pdf">Download PDF</a>.</p>
</embed>
</object>
</div>
<div class="modal-footer">
<button type="button" class="btn btn-secondary" data-dismiss="modal">Close</button>
<button type="button" class="btn btn-primary">Save changes</button>
</div>
</div>
</div>
</div>
<script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>
<script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.6.0/js/bootstrap.min.js"></script>
</body>
</html>
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