Well, your question prompted me to look around for a program and I discovered that I already had Recoll
installed. Wish I had known that, would have been using it. So, I have no real experience with it yet, but it seems to work OK.
Link to product: Recoil, Full Text Search for Linux
It is based on the very strong Xapian search engine library, for which it provides a powerful text extraction layer and a complete, yet easy to use, Qt graphical interface.
Recoll is free, open source, and licensed under the GPL. The current version is 1.16.2 (Release notes).