Hello Antonio, you can go to about:config (type in URL bar) and change pdfjs.disabled to false. PDF display is disabled mainly for performance reasons, but you can try it and see if it's good enough for you. It will be faster in future versions of TFF. Right now it's rather slow (depending on the complexity of the document), and some documents may not display properly. I've been using it for at least a year and found it usable enough to keep it enabled.