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.
RenderingStates,
  ScrollMode,
  SidebarView,
  SpreadMode,
  TextLayerMode,
} from "./ui_utils.js";
import { AppOptions, OptionKind } from "./app_options.js";
import { AutomationEventBus, EventBus } from "./event_utils.js";
import {
  build,
  createPromiseCapability,
  getDocument,
  getFilenameFromUrl,
  getPdfFilenameFromUrl,
  GlobalWorkerOptions,
  InvalidPDFException,
  isPdfFile,
  loadScript,
  MissingPDFException,
  OPS,
  PDFWorker,
  shadow,
  UnexpectedResponseException,
  UNSUPPORTED_FEATURES,
  version,
} from "pdfjs-lib";
import { CursorTool, PDFCursorTools } from "./pdf_cursor_tools.js";
import { LinkTarget, PDFLinkService } from "./pdf_link_service.js";
import { OverlayManager } from "./overlay_manager.js";
import { PasswordPrompt } from "./password_prompt.js";