WordPress 4.7 features enhanced pdf thumbnail support which is requires imagick. Fortunately its very easy to add imagick support to Turnkey Linux WordPress. Just start Webmin, go to System > Software Packages and add the php5-imagick Package from APT. Restart Apache and you're done. When adding PDF's to the Media Library thumbnails are generated. There is a plugin to regenerate thumbnails on old PDF's but I haven't tried that yet.

Thanks to turnkey for creating these useful distributions.

Bjarne