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 (!permissions) {
      return;
    }
    if (!permissions.includes(PermissionFlag.COPY)) {
      this.viewer.classList.add(ENABLE_PERMISSIONS_CLASS);
    }
    if (
      !permissions.includes(PermissionFlag.MODIFY_ANNOTATIONS) &&
      !permissions.includes(PermissionFlag.FILL_INTERACTIVE_FORMS)
    ) {
      if (this.#annotationMode === AnnotationMode.ENABLE_FORMS) {
        this.#previousAnnotationMode = this.#annotationMode; 
        this.#annotationMode = AnnotationMode.ENABLE;
      }
    }
  }
  #onePageRenderedOrForceFetch() {
    
    
    
    
    
    
    
    
    
    
    if (
      document.visibilityState === "hidden" ||