Cannot open `pg_hba.conf` in pgAdmin III
It's not clear what is the point in doing that, but if you're an admin you can do chmod o+r pg_hba.conf to let a non-admin read it.

There are two ways in pgAdmin3 to access those files:


by reading the file system (if it runs on the same host than the server). This corresponds to the File-&gt;Open *.conf commands. With that method, permission-wise, pgAdmin is not special, it's like a text editor or a pager. 
through the DB connection, with Tools-&gt;Server configuration. This requires the adminpack contrib module installed server-side. In the case of Ubuntu, I'm not sure it would work because the configuration files are not inside the data directory below /var, they're under /etc/... and the server-side functions involved are not supposed to reach anything outside the data directory. But if/when this method does work, it's the postgres process that reads the files, so the permissions shown in the question are good enough.