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 disableAutoFetch = this.pdfDocument
        ? this.pdfDocument.loadingParams.disableAutoFetch
        : AppOptions.get("disableAutoFetch");
      if (disableAutoFetch && percent) {
        if (this.disableAutoFetchLoadingBarTimeout) {
          clearTimeout(this.disableAutoFetchLoadingBarTimeout);
          this.disableAutoFetchLoadingBarTimeout = null;
        }
        this.loadingBar.show();
        this.disableAutoFetchLoadingBarTimeout = setTimeout(() => {
          this.loadingBar.hide();
          this.disableAutoFetchLoadingBarTimeout = null;
        }, DISABLE_AUTO_FETCH_LOADING_BAR_TIMEOUT);
      }
    }
  },
  load(pdfDocument) {
    this.pdfDocument = pdfDocument;
    pdfDocument.getDownloadInfo().then(({ length }) => {
      this._contentLength = length; 
      this.downloadComplete = true;
      this.loadingBar.hide();
      firstPagePromise.then(() => {
        this.eventBus.dispatch("documentloaded", { source: this });
      });
    });