I don't know about WKHTML, but maybe I can help with getting PhantomJS to work. Do you use the latest PhantomJS binary from the official website (https://phantomjs.org/download.html)? In my experience the PhantomJS packages from the distribution repositories (specifically Ubuntu) do not work for some reason.
Also please double check the paths to the PhantomJS binary in "Administration -> System Settings and Maintenance -> PDF Generation" (usually either "/usr/bin/phantomjs" or "/usr/local/bin/phantomjs").
Also please double check the paths to the PhantomJS binary in "Administration -> System Settings and Maintenance -> PDF Generation" (usually either "/usr/bin/phantomjs" or "/usr/local/bin/phantomjs").