What this add-on does is it basically calls the reader mode which is integrated into Firefox using a provided protocol. In Pale Moon, however, the reader mode is most likely not meant to be accessible, neither by users nor by add-ons.
One therefore first has to tweak the Pale Moon source code in a way that the reader mode becomes accessible.
But there already exist add-ons that mimic the behaviour of the Firefox reader mode and work for Pale Moon, for example “Enjoy Reading”  which also specifically targets Pale Moon . Another one working for Pale Moon is “TidyRead” , which runs only in compatibility mode right now, though.