A PDF add-on for the built-in eBook reader of MiXplorer file manager. MiXplorer v6.57.6+ has to be installed first and it won't be visible in your launcher. It can be uninstalled from Android or MiXplorer settings.

The Silver version doesn't need this add-on. You can download it from here:
https://play.google.com/store/apps/details?id=com.mixplorer.silver