Tom,
Presentations are created in HTML so you should be able to a standard HREF tag.
Try this and see if works for you;
<a href="path to your file">Click here</a>
*add the code to your presentation as a text element
Of course you can add many other HTML attributes to the following HREF tag if you need to. If you don't have any luck. I will take a peek at making this work on Sunday.
Christopher