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 && this.downloadComplete) {
      return;
    }
    throw new Error("PDF document not downloaded.");
  },
  async download() {
    const url = this._downloadUrl,
      filename = this._docFilename;
    try {
      this._ensureDownloadComplete();
      const data = await this.pdfDocument.getData();
      const blob = new Blob([data], { type: "application/pdf" });
      await this.downloadManager.download(blob, url, filename);
    } catch (reason) {
      
      
      await this.downloadManager.downloadUrl(url, filename);
    }
  },
  async save() {