What can clients do without having a Hightail account?
When your working with people who don't have a hightail account it is perfectly fine as they still can access the web without one.
Clients are able to view any shared documents as long as you the user have the Space setting listed for public viewing. This makes sure that anyone with the link can look at the files you have placed inside of a Space.
Aside from viewing the Space they are able to download any sent files and attach files to your uplink.