form is the simplest way, but you could do a link as well.
In the case of a link, you would want to pass the records ID as a querystring variable. then create a recordset to lookup the file name based on the ID and use the lookup recordsets fileName column in the download file behviaor.
you will also want to change the trigger from form post to query string being set. On the bindings panel, click the plus button and select URL variable to create a dynamic binding for the querystring, then in the trigger, click the lightning bolt and select the URL variable.