Express provides a handy method to transfer a file as attachment: Response.download().

Once a user hits a route that sends a file using this method, browsers will prompt the user for download.

The Response.download() method allows you to send a file attached to the request, and the browser instead of showing it in the page, it will save it to disk.

app.get('/', (req, res) => res.download('./file.pdf'))

In the context of an app:

const express = require('express')
const app = express()

app.get('/', (req, res) => res.download('./file.pdf'))
app.listen(3000, () => console.log('Server ready'))

You can set the file to be sent with a custom filename:

res.download('./file.pdf', 'user-facing-filename.pdf')

This method provides a callback function which you can use to execute code once the file has been sent:

res.download('./file.pdf', 'user-facing-filename.pdf', (err) => {
  if (err) {
    //handle error
    return
  } else {
    //do something
  }
})