That plugin hasn't been supported in quite a while, it seems. Some people have added comments about code changes to get it working on later versions.
https://moodle.org/plugins/mod_pdfjsfolder/versions
It's not a 1:1 replication of functionality, but note that if you add a pdf as a file resource, there's an option under 'Appearance' -> Display to 'embed' the file. This will use the browser's PDF renderer, which these days is pretty reliable (in Chrome/Edge, Safari and Firefox).
https://moodle.org/plugins/mod_pdfjsfolder/versions
It's not a 1:1 replication of functionality, but note that if you add a pdf as a file resource, there's an option under 'Appearance' -> Display to 'embed' the file. This will use the browser's PDF renderer, which these days is pretty reliable (in Chrome/Edge, Safari and Firefox).