Unfortunately, one may not feel comfortable to let google, microsoft or even apple view the very heart of your files, especially as they also offer a network index sharing function. Might be really wise to be a little bit paranoiac these days. Not that I have a big deal to hide, but just for a good feeling.
It appears to me, there are not so many high quality open source replacements, or at least, easily usable ones. I found DocFetcher to be quite good at doing the basic job, although it won't certainly win any design award. It's text-only search, no mp3/image/video attribute search, and has also a primitive but effective preview function, working also with pdfs, highlighting the search query as you can see here (click to enlarge):
