How do I prevent mobile devices from automatically downloading a PDF on my website?

So I've got a part on my website where I show a pdf in an iframe. This works perfectly fine on computers. However, some mobile devices start downloading the pdf immediately when they load the website.

This is the code I use.

<div class="showbox" style="display: none;">
    <div class="embed-container">
        <iframe src="/images/pdf-file.pdf" style="border:none;"></iframe>
    </div>
</div>

So I just want the mobile users to be able to visit the website without automatically downloading the pdf.

Any ideas how to pull this off?