I use a Windows program called Everything, from voidtools.com. It is not what you want (it's Windows only, and folder and filenames only), but it is fast and flexible, so I used it as a starting point to see what else was available. A search at alternativeto.net for Everything, for Linux, gives 5 total hits. Search on Alternativeto.net
Of these, all 5 are open source. 3 claim to search contents as well: Regain, SearchMonkey, and Tracker. A fourth, Catfish, is unclear as to whether it searches content; it's described as a search front end.
The link to the regain website on alternativeto.net is obsolete, I believe. The one I found that works is: Regain website Note that is cross platform. Claimed to be very fast because it uses an index. On Windows, it has to be installed. Don't know about Linux.
The link to the SearchMonkey website is: SearchMonkey website It is also cross platform. It does not require installation on Windows. It does not seem to use an index, so may not be faster than grep.
I have not used Regain, but did try SearchMonkey. Seems OK. Again, on Windows.
Edit2 (1/19/16): I rediscovered another program that I had previously downloaded, DocFetcher. I ran some tests on it under windows and I believe it does what you want. It is free, open source, and cross platform. It will search inside files and display the relevant plain text. It supports multiple file types, including the Outlook pst. It will generate an index, with user control of the path being indexed, so you could index just your pst file. The indexing may take some time, but it is then quite fast. It needs Java. It's available at: DocFetcher on Sourceforge Its website, also on Sourceforge, is at DocFetcher website. The comments on Sourceforge are generally very positive.