Google Desktop was a good search tool to find documents on your local computer that contained a particular text. It was useful because it indexed all types of documents, not just text files, like MS Office, Open Office and PDF.
Since Google Desktop is discontinued, I have found that a good replacement is Recoll. It works similarly and supports many more document formats, including DjVu. It can be a good tool to search your library of documents and papers.
$ sudo apt install recoll
When you start Recoll for the first time, you can set the directory containing your documents and the daily time to run a Cron job to index new document files. So, the first time you use Recoll, you will need to wait until it has indexed your documents for the first time.
Usage is straightforward: type in the text or phrase you are looking for and it shows the documents which have it and some excerpts from them to make it easy to pick what you want.
Recoll learns how to index a type of file by using a helper. You can see which helpers are needed for the documents in your directory, by clicking File -> Show missing helpers.
If you have MS Word documents, install Antiword:
$ sudo apt install antiword
$ sudo apt install unrtf
$ sudo pip install epub
Tried with: Recoll 1.17.3 and Ubuntu 14.04