Please verify with us first that your website is setup to do this. Otherwise it will not work.
If it is setup, go the page you want to insert the PDF into and click the EDIT button at top right of page.
Click the INSERT IMAGE button and upload your PDF file there in the media manager.
Click INSERT at bottom right of the media manager popup.
A piece of code will be inserted into the page that will represent this PDF.
Save the page at the top right.