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.
const event = document.createEvent("CustomEvent");
      event.initCustomEvent("webviewerloaded", true, true, {
        source: window,
      });
      try {
        
        
        
        parent.document.dispatchEvent(event);
      } catch (ex) {
        
        
        console.error(`webviewerloaded: ${ex}`);
        document.dispatchEvent(event);
      }
    }
    PDFViewerApplication.run(config);
  }
}
document.blockUnblockOnload?.(true);
if (
  document.readyState === "interactive" ||
  document.readyState === "complete"
) {
  webViewerLoad();
} else {
  document.addEventListener("DOMContentLoaded", webViewerLoad, true);
}
export { PDFViewerApplication, AppOptions as PDFViewerApplicationOptions };