It appears that the official chrome webstore extension by Rob W is outdated. It was last updated in 2019. 

MOZCENTRAL_DIR + "browser/locales/en-US/pdfviewer/",
        FIREFOX_CONTENT_DIR = EXTENSION_SRC_DIR + "/firefox/content/";
      const MOZCENTRAL_WEB_FILES = [
        ...COMMON_WEB_FILES,
        "!web/images/toolbarButton-openFile.svg",
      ];
      
      rimraf.sync(MOZCENTRAL_DIR);
      return merge([
        createMainBundle(defines).pipe(
          gulp.dest(MOZCENTRAL_CONTENT_DIR + "build")
        ),
        createScriptingBundle(defines).pipe(
          gulp.dest(MOZCENTRAL_CONTENT_DIR + "build")
        ),
        createSandboxExternal(defines).pipe(
          gulp.dest(MOZCENTRAL_CONTENT_DIR + "build")
        ),
        createWorkerBundle(defines).pipe(
          gulp.dest(MOZCENTRAL_CONTENT_DIR + "build")
        ),
        createWebBundle(defines, { defaultPreferencesDir: "mozcentral/" }).pipe(
          gulp.dest(MOZCENTRAL_CONTENT_DIR + "web")
        ),
        gulp
          .src(MOZCENTRAL_WEB_FILES, { base: "web/" })
          .pipe(gulp.dest(MOZCENTRAL_CONTENT_DIR + "web")),
        createCMapBundle().pipe(
          gulp.dest(MOZCENTRAL_CONTENT_DIR + "web/cmaps")
        ),