You’re right, this plugin is not actively maintained. WordPress introduced a built-in feature for generating PDF thumbnails in version 4.7, making this plugin obsolete, see https://core.trac.wordpress.org/ticket/31050 . As far as I know, it has not been tested in WordPress 5.
The next time you find yourself struggling to figure out how to use a free plugin that someone created so that you could spend more time drinking coffee and munching cookies, I suggest posting a friendly support request in the support thread to figure out where the problem is located.
I would delete my original review, but I find your response useful, and it may be useful to others.
