Recoll

A cross-platform desktop search engine that can search through File metadata from various formats including HTML, text files, PDF, and more. It can be automated easily with the command line and it has a GUI available as well as various third-party frontends. Moreover, it has a Python 3 API that can extend Recoll for support to additional file formats, for example.

Like most web search engines, it searches through an index which has to be generated first (i.e., recollindex). With the file changes, the index has to be regenerated.

Recoll is an example of a developer-oriented application. It has extensive user documentation that lets you extend Recoll through your own. It features both a command line program and a graphical interface for a user-friendly option or a different way of navigation.