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.
this.l10n = this.externalServices.createL10n(
      typeof PDFJSDev === "undefined" || PDFJSDev.test("!PRODUCTION || GENERIC")
        ? { locale: AppOptions.get("locale") }
        : null
    );
    const dir = await this.l10n.getDirection();
    document.getElementsByTagName("html")[0].dir = dir;
  },
  /**
   * @private
   */
  _forceCssTheme() {
    const cssTheme = AppOptions.get("viewerCssTheme");
    if (
      cssTheme === ViewerCssTheme.AUTOMATIC ||
      !Object.values(ViewerCssTheme).includes(cssTheme)
    ) {
      return;
    }
    try {
      const styleSheet = document.styleSheets[0];
      const cssRules = styleSheet?.cssRules || [];