SpiderOak ONE and Groups allow you to share files with others via both expiring and unexpiring links. To create an expiring link, see Sharing a Single File. The link will expire after three days.
For unexpiring links, set up a ShareRoom and place your file in it. To share the entire contents of the ShareRoom, give out the link of the ShareRoom. To share just one file in the ShareRoom, open the ShareRoom in your browser, right click on the desired file, and select "Copy Link Location". The exact wording of this option will depend upon your browser. Then give out that link you just copied.