how to display a pdf file in modal window.

I don't think your button can work this way.
You are using Bootstrap and FontAwesome right?
Try this:

PHP Code:

<td><?= anchor('users/download/'.$row['markid'],'<i class="fa fa-eye"></i>');?></td> 


Typically, force_download will trigger your browser to ask the user whether he wants to save or open the document.