Hi Nigel
This should be an easy solve if you could supply a bit more information to assist us.
Can you tell us what version of Creator you are using and what type of publication you are making, for example HTML5
I'm going to assume that you have Creator 9 and are making a HTML5 publication.
Add a left mouse click to your thumbnail
Add a Launch Webpage action
Type in the web address including the file name that is to be downloaded; eg
Http://www.thisaddress.com/files/musings.pdfPublish, upload to your website and test.
But I'm assuming and without the vital information to assist us that we need, it is hard to accurately assist you.
You can add your details using the forum User Control Panel. (Top right hand side of screen) if you like.
Cheers
Graham