In ROS there is the fetch command, that can both download and upload files:
https://wiki.mikrotik.com/wiki/Manual:Tools/Fetch
As always there is no practical example in the above page for your use case.
You will need I believe a FTP server:
http://forum.mikrotik.com/t/export-and-download/162899/1
But doesn’t Webfig work for this?
A thread with more details on scp usage:
http://forum.mikrotik.com/t/downloading-and-uploading-using-scp/170608/1