I wouldn’t use a long running orchestration. You would need to implement some message at an interval to start this orchestration, you never know when it may stop. You might as well use this trigger to initiate a short running orchestration to down load the file.
There are a number of scheduling patterns available: http://geekswithblogs.net/michaelstephenson/archive/2008/05/16/122203.aspx
I use the Scheduled Task Adapter here: http://biztalkscheduledtask.codeplex.com/
Another consideration is the Http method being used. Are you using a GET or a POST? If you can browse to the file using a browser then it is probably a GET. The HTTP adapter does not support Http GET only Http POST. The scheduled task adapter has an HttpDownload task that will perform an Http GET to download a file.