My suggestion would be to avoid using popups or opening a new tab since it seems unnecessary based on your query. Instead, you could consider placing the hidden content within your div (for instance, below your "read more" link) and utilize jQuery to slide it up/down upon clicking the 'Readmore' link.
Check out the DEMO here
Here is some sample JavaScript code:
$('.readmore').click(function(){
$('.hidden').slideUp(200);
var divToShow = $(this).parent().find('.hidden');
divToShow.slideDown(300);
});
And here is an example of the HTML structure:
<div class="content">
<p>Lorem ipsum dolor sit amet, consectetur adipiscing elit.</p>
<span class="readmore">Read more</span>
<div class="hidden">This is some hidden content.</div>
</div>
<div class="content">
<p>Lorem ipsum dolor sit amet, consectetur adipiscing elit.</p>
<span class="readmore">Read more</span>
<div class="hidden">This is some hidden content.</div>
</div>
<div class="content">
<p>Lorem ipsum dolor sit amet, consectetur adipiscing elit.</p>
<span class="readmore">Read more</span>
<div class="hidden">This is some hidden content.</div>
</div>
<div class="content">
<p>Lorem ipsum dolor sit amet, consectetur adipiscing elit.</p>
<span class="readmore">Read more</span>
<div class="hidden">This is some hidden content.</div>
</div>