I there a way to prevent external user to access the saves searches? OR best to allow him to see only specific saved searches?
the problem is that anything that is done in a saved search can also be done in a regular search so you would have to have access controls for every possible search. Searches won't return results for documents that users don't have permission to access. If you need to make sure certain documents won't appear in search results then you need to set document permissions appropriately. The simplest search to do is go to right-click the Documents folder, select Advanced Search, click Search Form, then just click OK. That returns every document in the datasource that the user has access to. If you aren't protecting against this search then your documents aren't secure.