The prebuilt files include `web/pdf_viewer.js` with some viewer APIs. Those are missing from type declarations.
gulp.task(
  "mozcentraldiff",
  gulp.series(
    "mozcentral",
    "mozcentralbaseline",
    function createMozcentralDiff(done) {
      console.log();
      console.log("### Creating mozcentral diff");
      
      
      
      
      
      rimraf.sync(MOZCENTRAL_BASELINE_DIR + "*");
      gulp
        .src([BUILD_DIR + "mozcentral/**/*"])
        .pipe(gulp.dest(MOZCENTRAL_BASELINE_DIR))
        .on("end", function () {
          safeSpawnSync("git", ["add", "-A"], { cwd: MOZCENTRAL_BASELINE_DIR });
          const diff = safeSpawnSync(
            "git",
            ["diff", "--binary", "--cached", "--unified=8"],
            { cwd: MOZCENTRAL_BASELINE_DIR }
          ).stdout;
          createStringSource(MOZCENTRAL_DIFF_FILE, diff)
            .pipe(gulp.dest(BUILD_DIR))
            .on("end", function () {
              console.log(
                "Result diff can be found at " +
                  BUILD_DIR +