Hopefully you already know that you can create an Upload Button which lets your users upload a file or image to a collection.
But did you know that you can also let your users download uploaded documents? Without code?
To do this:
Select an element that you can add a link to (buttons, tables, images, galleries)
Go to their Connect to Data panel and under "Link Connects to" set the link to the field in your collection where you stored the document
When a user clicks the linked element, the document downloads automatically - and that's it.
Note that you can only download documents using this method, not images.