Well, the files aren't related to a place in the page tree, strictly speaking. But perhaps you can dynamically set the SearchLocations property to the unified file system path you want to search. If it is the page files perhaps you can get the path to the page's files using the PageData's GetPageDirectory method.
I'm having problems when searching for files, the problem is this:
I am using different search start pages (using .PageLink) to limit results, this works as it should, but when I am searching for files using the EPiServer CMS Indexing Service and Ifilter, it completely disregards the start page/.PageLink setting.
SearchDataSource.PageLink = new PageReference(int.Parse(SearchRoot.SelectedValue));
... Where SearchRoot is a drop down box populated with page Id's.