How to make a download link also activate modal pop up message?

first time asker here.

I'm trying to make a download button not only serve the file download but also cause a modal pop up to be activated giving the user simple instructions on downloading the file.

I'm using what I've researched online as this:

.modalDialog {
  position: fixed;
  font-family: Arial, Helvetica, sans-serif;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  background: rgba(0, 0, 0, 0.8);
  z-index: 99999;
  opacity: 0;
  -webkit-transition: opacity 400ms ease-in;
  -moz-transition: opacity 400ms ease-in;
  transition: opacity 400ms ease-in;
  pointer-events: none;
}
.modalDialog:target {
  opacity: 1;
  pointer-events: auto;
}
.modalDialog > div {
  width: 400px;
  position: absolute;
  bottom: -125px;
  left: 10px;
  margin: 10% auto;
  padding: 5px 20px 13px 20px;
  border-radius: 10px;
  /*background: #fff;
        background: -moz-linear-gradient(#fff, #999);
        background: -webkit-linear-gradient(#fff, #999);
        background: -o-linear-gradient(#fff, #999);*/
}

<div id="openModal" class="modalDialog">
  <div>
    <a href="#close" title="Close" class="close">X</a>
    <img src="http://ift.tt/1ABLuSc" />
  </div>
</div>

<a href="#openModal" onclick="window.open('pathtodownloadfile');" class="download-now">Download</a> 

It works in that it causes the modal pop up to activate, and it opens a new tab for the file to be served from.

Is there any way to do this so that the file can be served without opening a new window? I've seen sites have a download button where when pressed, a small instructional modal pop up occurs and helps the user to download/install the file all within the same page, no second window needed..

What am I missing?