Are you talking about an experience where the admin logs into the CMS and can add a link to a PDF document? If so this is not supported by default with Power CMS but it can be done by making some adjustments to the HTML Editor resources that are used in PowerCMS. Here are some links to posts that discuss the steps that you can take to allow this:
Uploading a PDF
showthread.php?p=52355&posted=1#post52355
Allowing banned files
showthread.php?t=9809