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.
noContextMenuHandler,
} from "./ui_utils.js";
import { AnnotationEditorType } from "pdfjs-lib";
const PAGE_NUMBER_LOADING_INDICATOR = "visiblePageIsLoading";
/**
 * @typedef {Object} ToolbarOptions
 * @property {HTMLDivElement} container - Container for the secondary toolbar.
 * @property {HTMLSpanElement} numPages - Label that contains number of pages.
 * @property {HTMLInputElement} pageNumber - Control for display and user input
 *   of the current page number.
 * @property {HTMLSelectElement} scaleSelect - Scale selection control.
 *   Its width is adjusted, when necessary, on UI localization.
 * @property {HTMLOptionElement} customScaleOption - The item used to display
 *   a non-predefined scale.
 * @property {HTMLButtonElement} previous - Button to go to the previous page.
 * @property {HTMLButtonElement} next - Button to go to the next page.
 * @property {HTMLButtonElement} zoomIn - Button to zoom in the pages.
 * @property {HTMLButtonElement} zoomOut - Button to zoom out the pages.
 * @property {HTMLButtonElement} viewFind - Button to open find bar.
 * @property {HTMLButtonElement} openFile - Button to open a new document.
 * @property {HTMLButtonElement} presentationModeButton - Button to switch to
 *   presentation mode.
 * @property {HTMLButtonElement} editorNoneButton - Button to disable editing.
 * @property {HTMLButtonElement} editorFreeTextButton - Button to switch to
 *   FreeText editing.
 * @property {HTMLButtonElement} download - Button to download the document.
 * @property {HTMLAnchorElement} viewBookmark - Button to obtain a bookmark link
 *   to the current location in the document.
 */
class Toolbar {