How can I use Dialogflow fulfillment and Google Cloud Storage to send a user a PDF file triggered by an intent?

Question

I wanted to use dialogflow fulfillment and google cloud storage to set up an intent in the inline editor so that when a user triggers the intent a pdf file is sent to them. But unfortunately it does not work for me yet. I would be so grateful for help.

This is what I tried:

function sendPDF(agent) {
    const pdfBucket = 'mybucket'; 
    const pdfFileName = 'file.pdf'; 
    const pdfUrl = `https://storage.googleapis.com/${pdfBucket}/${pdfFileName}`;

    agent.add('Here is your file:');
    agent.add(new Card({
      title: 'Download PDF',
      buttonText: 'Download',
      buttonUrl: pdfUrl
    }));

    return Promise.resolve();
  }

intentMap.set('myIntent', sendPDF);