It's weird that Alfresco Share uses an OR search by default. If we want to find files related to the budget of the marketing department we would have to search for
budget AND marketing
There's an article and a bug report with information on how to change the default behavior to an AND search, but they require you to modify the Java code, which is quite complicated.
As long as this problem is not fixed in Alfresco, you may find it easier to make some JavaScript changes. Just edit file /opt/alfresco-4.2.b/tomcat/webapps/share/components/search/search.js and modifiy the _performSearch function as following:
