The prebuilt files include `web/pdf_viewer.js` with some viewer APIs. Those are missing from type declarations.
});
      return merge([
        buildDefaultPreferences(defines, "chromium/"),
        createTemporaryScriptingBundle(defines),
      ]);
    },
    function createChromium() {
      console.log();
      console.log("### Building Chromium extension");
      const defines = builder.merge(DEFINES, {
        CHROME: true,
        SKIP_BABEL: false,
      });
      const CHROME_BUILD_DIR = BUILD_DIR + "/chromium/",
        CHROME_BUILD_CONTENT_DIR = CHROME_BUILD_DIR + "/content/";
      
      rimraf.sync(CHROME_BUILD_DIR);
      const version = getVersionJSON().version;
      return merge([
        createMainBundle(defines).pipe(
          gulp.dest(CHROME_BUILD_CONTENT_DIR + "build")
        ),
        createWorkerBundle(defines).pipe(
          gulp.dest(CHROME_BUILD_CONTENT_DIR + "build")
        ),
        createSandboxBundle(defines).pipe(
          gulp.dest(CHROME_BUILD_CONTENT_DIR + "build")
        ),
        createWebBundle(defines, { defaultPreferencesDir: "chromium/" }).pipe(
          gulp.dest(CHROME_BUILD_CONTENT_DIR + "web")