I recommend to install and use Recoll Lens:

Recoll is a full text search desktop tool which indexes the contents
of many file formats including OpenOffice, MS Office, PostScript, MP3
and other audio files, JPEG and more. Besides regular searches, Recoll
also lets you perform some advanced operations like searching for the
author, file size, file format as well as operators like "AND" or
"OR".

Unity Recoll Lens lets you use Recoll from Dash, without having to
open any additional GUI. The lens comes with a few filters like Text,
Spreadsheet, Presentation, Media or Message, but for more advanced
searches, you'll have to manually enter the Recoll search query.
Examples: author:"john doe" metallica OR megadeth /2007 (all documents
from 2007 or older) dir:/path/to/dir (filters content from
/path/to/dir directory).