Hi,
Currently, if you have the URL of the PDF (you can get this by clicking on the LO in the Workspace, clicking Properties, going to the Media and Quota tab and right clicking on the relevant item's url) then you can just insert:
<a href="YOUR_URL_HERE">NAME OF PDF FILE</a>
into pretty much any text area and you will get a link. Customise the bold bits in the link tag.
Where this falls down is if you duplicate the LO and/or export. The URL in the resulting LO will still link back to the URL that you copied. As long as the file is still there then you should be ok but it can cause cross domain issues or offline problems.
We are currently working on a new editor that should take these issues into consideration and format them in the same way as other media so that there are relative to the current LO.
John