Hello.
I am not quite sure what do you mean by "backend" document. How it differs with a standard document, please? Even folder (
cms.folder) is just a special kind of a document, file (
cms.file) as well. If you want to exclude some documents from search index, or search specific document type(s) only (e.g. skip
cms.folder and
cms.file document types), you can set it on smart search index´ level, as it is described here:
http://devnet.kentico.com/docs/devguide/smart_search_definign_index_content.htm.
Best Regards,
Radek Macalik