==== limit search results by extension ====
Enable the google mode
  libgen.li -> Google mode -> drag to right
search using the "ext" field. For example, to search for pdf files
  query ext:pdf

Notes:
  * This works on libgen.li but not on libgen.rs

Ref:
  * https://www.reddit.com/r/libgen/comments/j9fjbo/can_you_search_for_books_of_a_certain_format/ - where I found the answer
  * To see the "Google mode" syntax, click on the "?" after "Google mode" checkbox