The current implementation for Drupal 9 requires manual download and installation of the Mozilla pdf.js library old school (i.e. below the /libraries folder). This is no longer the best practice for Drupal modules requiring third party libraries.
The library is available from packagist: https://packagist.org/packages/clean-composer-packages/pdf-js
Use composer to handle the dependency on pdf.js
composer.json to the project.composer.json.Active
1.0
Code
Duplicate of https://www.drupal.org/project/pdf/issues/2805053 🐛 Required PDF.js library is not installed when using composer RTBC