We could use:
https://github.com/mozilla/pdf.js/blob/c3d08580627471432a6ad0b2fdb948df64126afa/src/display/api.js#L929

and look at bit 4.

The following pdf:
https://orimi.com/pdf-test.pdf
is not editable in neither Acrobat nor Edge nor Foxit and nor Okular.
/* Copyright 2012 Mozilla Foundation
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
import {
  animationStarted,
  apiPageLayoutToViewerModes,
  apiPageModeToSidebarView,
  AutoPrintRegExp,
  DEFAULT_SCALE_VALUE,
  getActiveOrFocusedElement,
  isValidRotation,
  isValidScrollMode,
  isValidSpreadMode,
  noContextMenuHandler,
  normalizeWheelEventDirection,
  parseQueryString,
  ProgressBar,
  RendererType,