The prebuilt files include `web/pdf_viewer.js` with some viewer APIs. Those are missing from type declarations.
gulp.task(
  "mozcentralbaseline",
  gulp.series(createBaseline, function createMozcentralBaseline(done) {
    console.log();
    console.log("### Creating mozcentral baseline environment");
    
    rimraf.sync(BASELINE_DIR + BUILD_DIR);
    const workingDirectory = path.resolve(process.cwd(), BASELINE_DIR);
    safeSpawnSync("gulp", ["mozcentral"], {
      env: process.env,
      cwd: workingDirectory,
      stdio: "inherit",
    });
    
    rimraf.sync(MOZCENTRAL_BASELINE_DIR);
    mkdirp.sync(MOZCENTRAL_BASELINE_DIR);
    gulp
      .src([BASELINE_DIR + BUILD_DIR + "mozcentral/**/*"])
      .pipe(gulp.dest(MOZCENTRAL_BASELINE_DIR))
      .on("end", function () {
        
        safeSpawnSync("git", ["init"], { cwd: MOZCENTRAL_BASELINE_DIR });
        safeSpawnSync("git", ["add", "."], { cwd: MOZCENTRAL_BASELINE_DIR });
        safeSpawnSync("git", ["commit", "-m", '"mozcentral baseline"'], {
          cwd: MOZCENTRAL_BASELINE_DIR,
        });
        done();
      });
  })
);