Rather than copying an exec into moodle code directory ... which might get deleted when upgrading to 3.4.higher or 3.5 depending upon how one update/upgrades (git is best, IMHO), suggest, if you can/are able, check out:
http://php.net/manual/en/ini.core.php#ini.open-basedir
And include /usr/bin/
Are there other paths that show 'x' ... like du? or python? Since you might be looking at annotation seutp with unoconv, which is a python script, might be a good idea to keep things 'standard' instead of 'work-arounds' that might come back to bite. ;) My 2 cents, of course.
'spirit of sharing', Ken