访问指定 URL，生成 PDF。
- https://blog.risingstack.com/pdf-from-html-node-js-puppeteer/ 3种方式生成 PDF
- https://stackoverflow.com/questions/33482688/initiate-file-download-with-koa 如何使用 koa 返回文件
- https://ourcodeworld.com/articles/read/1106/how-to-solve-puppeteer-timeouterror-navigation-timeout-of-30000-ms-exceeded resolve 30s timeout