ExpressJS and PDFKit - generate a PDF in memory and send to client for download

In my api router, there is a function called generatePDF which aims to use PDFKit module to generate a PDF file in memory and send to client for download instead of displaying only.

In api.js:

var express = require('express');
var router = express.Router();

const PDFDocument = require('pdfkit');

router.get('/generatePDF', async function(req, res, next) {
    var myDoc = new PDFDocument({bufferPages: true});
    myDoc.pipe(res);
    myDoc.font('Times-Roman')
         .fontSize(12)
         .text(`this is a test text`);
    myDoc.end();
    res.writeHead(200, {
        'Content-Type': 'application/pdf',
        'Content-disposition': 'attachment;filename=test.pdf',
        'Content-Length': 1111
    });
    res.send( myDoc.toString('base64'));
});

module.exports = router;

This does not work. The error message is (node:11444) UnhandledPromiseRejectionWarning: Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client.

How can I go about fixing the issue and getting it work?

Also, a relevant question would be how I can separate the business logic of PDF generation from the router and chain them up?

Complete solution.

var express = require('express');
var router = express.Router();

const PDFDocument =  require('pdfkit');

router.get('/generatePDF', async function(req, res, next) {
var myDoc = new PDFDocument({bufferPages: true});

let buffers = [];
myDoc.on('data', buffers.push.bind(buffers));
myDoc.on('end', () => {

    let pdfData = Buffer.concat(buffers);
    res.writeHead(200, {
    'Content-Length': Buffer.byteLength(pdfData),
    'Content-Type': 'application/pdf',
    'Content-disposition': 'attachment;filename=test.pdf',})
    .end(pdfData);

});

myDoc.font('Times-Roman')
     .fontSize(12)
     .text(`this is a test text`);
myDoc.end();
});

module.exports = router;