Based on this http://servergrrl.blogspot.com/2008/02/how-to-use-wss-30-to-search-more-than.html blog, I've created a domain account for testing and sets the permission at the side level as well as Document Library to allow specific user (DOMAIN/TestUser1) allow accessing this folder.
Now, it's interesting that the user can see the document by drilling though BUT it won't be able to search and the document couldn't find it. But if I tested with the site owner (both primary and secondary), it works nicely.
What did I do wrong then? It must be related on permission but I cound't work out what it is.
BTW ... I am using WSS not MOSS 2007.
Thanks