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: