You could try this code just put in your src url in between the quotes. This should give you a window that expands and contracts with the browser width in proportion

<div style="position: relative; padding-bottom: 56.25%; padding-top: 0px; height: 0; overflow: hidden;">
<iframe style="position: absolute; top: 0; left: 0; width: 100%; height: 100%;" src="" ></iframe>
</div>

if you want the object slightly smaller (60% width) you could try this:

<div style="position: relative; padding-bottom: 33.75%; padding-top: 0px; overflow: hidden; width: 60%; height: 60%; left: 20%; right: 20%;">
<iframe style="position: absolute; top: 0; left: 0; right: 0; width: 100%; height: 100%;" src="" ></iframe>
</div>

I usually use this code to embed video but it should work with your PDF