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.
if (!this.pdfDocument) {
      return;
    }
    const viewer = this.viewer,
      pages = this._pages;
    if (this._scrollMode === ScrollMode.PAGE) {
      this.#ensurePageViewVisible();
    } else {
      
      viewer.textContent = "";
      if (this._spreadMode === SpreadMode.NONE) {
        for (const pageView of this._pages) {
          viewer.append(pageView.div);
        }
      } else {
        const parity = this._spreadMode - 1;
        let spread = null;
        for (let i = 0, ii = pages.length; i < ii; ++i) {
          if (spread === null) {
            spread = document.createElement("div");