we have done this on our site by using a customvar and a button on the extended information page for each product (but you could include this wherever you want)
To do this, go to Advanced | Custom Properties and add a new property for your download.
Go to your product and in the Properties tab add a new line. Select your custom field, add the file name as the value, check Use as CUSTOMVAR and File name boxes
On the template page you wish your button/link to appear add your link code, for example (where your custom field is called PAGE):
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