When a file is uploaded within the CMS File Library, it is given a URL which then is available online/public. Your CMS page may require a login to access the file, but the file itself is not restricted by the login. 
Search engines are third-party software outside of Sphere. As a result, their crawl process is out of the scope of Sphere and the file cannot be prevented from being searched and/or opened if someone types its name in a search engine. 

However, you can secure a file on a CMS webpage by password protecting it in the third-party program from which it originated (example: Adobe, Microsoft Word, Microsoft Excel, and other third-party programs). Once this is done, you can give the file's login to the appropriate users who will be accessing it.