In case of Office documents i-filters are already provided with SharePoint to index content of documents while crawling. I suppose in SharePoint 2010 administrator must have been installed the iFilters for PDF files which is free to download and install. You would need to remove those iFilters, however bear in mind that such operation affects the entire farm.
Properties exposed via Search Core Results (change XML properties via Edit web part), after being promoted from crawled properties (automatically indexed properties from columns from content types) to search managed properties in SPCA - same concepts apply to SharePoint 2007/2010.
You could alternativelly (however that would mean entirelly hiding them from search) to remove confidential content by using library settings and un-check where where it says to make content available for search. Other alternative is also to modify your search scopes to remove those documents (or extensions) via rules.