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._saveInProgress) {
      return;
    }
    this._saveInProgress = true;
    await this.pdfScriptingManager.dispatchWillSave();
    const url = this._downloadUrl,
      filename = this._docFilename;
    try {
      this._ensureDownloadComplete();
      const data = await this.pdfDocument.saveDocument();
      const blob = new Blob([data], { type: "application/pdf" });
      await this.downloadManager.download(blob, url, filename);
    } catch (reason) {
      
      
      console.error(`Error when saving the document: ${reason.message}`);
      await this.download();
    } finally {
      await this.pdfScriptingManager.dispatchDidSave();
      this._saveInProgress = false;
    }
  },
  downloadOrSave() {
    if (this.pdfDocument?.annotationStorage.size > 0) {
      this.save();
    } else {
      this.download();
    }
  },
  fallback(featureId) {