It is built in in stable (at least it seems to be in the stable version in the Arch repos), you just have to enable it in about:config. If I remember correctly

It is built in in stable (at least it seems to be in the stable version in the Arch repos), you just have to enable it in about:config. If I remember correctly 'browser.preferences.inContent' has to be set to true and 'pdfjs.disabled' has to be set to false.