When I put ' on ' in the Search window I got everything with 'on' for example contributor, contents, section, on etc but I am searching ' on '?

Currently, when you input some spaces before the keywords in the search window, Foxit Reader will just automatically filter out the spaces from the keywords in the search window during searching. So when you input put ' on ' in the Search window, the search effect is equivalent to searching 'on'. It is our existing search mechanism which is also used by other PDF viewer (such as adobe reader, PDF-Xchange Viewer, etc).