Hi,
The description is not used anywhere.
There is no option to activate it.
You say that you want to display the descriptions of files on the product page, but unless the files are free, you won't see the files on the product page. Then, you're saying that you're selling the files. So I'm not sure where you want to display the descriptions ?
On the order details page ?
If so, you will have to edit the file "show" of the view "order" via the menu Display->Views and add some simple custom code.
If you want them on the product page, that will be the file "show_block_product_files" of the view "product".