WP Job Manager is a popular, well written job board plugin with a thriving add-on library. WP Job Manager offers various search implementations for itself and add-ons. If you would like to enhance the keyword (only) aspect of those search fields, WP Job Manager Integration will do the trick.
WP Job Manager’s search includes a number of filters and location parameters. WP Job Manager Integration does not interfere with those and instead limits the results pool based on the search string instead.
Check out more information on the WP Job Manager Integration extension page: https://searchwp.com/docs/extensions/wp-job-manager-integration/