During installation (and update) via admin panel
the script create a sub-folder named pdf24 inside wp-content/plugin/pdf24-post-to-pdf
and the system don't activate it (and don't show pdf24 in plugin list).
I have solved moving the files from wp-content/plugins/pdf24-post-to-pdf/pdf24/ to wp-content/plugins/pdf24-post-to-pdf .
br
Menelao