As you may know, Recoll is an open source full-text search tool for Linux, developed in Qt. Among others, it has support for finding keywords inside text files (capable of reading all the popular document formats), email attachments, archive members, etc.
The latest version available is Recoll 1.21.4, which has been recently released, coming with changes and fixes.
Up to date packages are available via some third party PPA, so installing recoll on Ubuntu 15.10 Wily Werewolf, Ubuntu 15.04 Vivid Vervet, Ubuntu 14.04 Trusty Tahr, Linux Mint 17.x, Elementary OS 0.3 Freya and other Ubuntu derivative systems is easy.
All you have to do is add the pa to your system, update the local repository index and install the recoll package. Like this:
$ sudo add-apt-repository ppa:recoll-backports/recoll-1.15-on
$ sudo apt-get update
$ sudo apt-get install recoll
Optional, to remove recoll, do:
$ sudo apt-get remove recoll