Hi Michel,

You can refer this module, this module will add the functionality such that user can download the attachment from the website.

https://www.odoo.com/apps/modules/10.0/website_product_attachments/

I know this question is posted so long ago, may the answer helps some others who is  looking for the same.

Thanks

Hello Michel, if for example you have product that you set up in your back-end with the eCommerce, you can allow your customers to download a file after they buy a product, but configuring the digital product. 

A simpler and more direct version is to add a "click here" to download a file and you link it with a dropbox or a google doc.

Hope it helps!