https://devcenter.heroku.com/articles/hypdf#pdfunite

node_pdfunite.js

var FormData = require('form-data'),
    fs = require("fs");

var form = new FormData();
form.append('user', 'HYPDF_USER');
form.append('password', 'HYPDF_PASSWORD');
form.append('test', 'true');
form.append('bucket', 'hypdf_test');
form.append('key', 'hypdf_test.pdf');
form.append('public', 'true');
form.append('file1', fs.createReadStream('/path/to/file1.pdf'));
form.append('file2', fs.createReadStream('/path/to/file2.pdf'));
form.append('file3', fs.createReadStream('/path/to/file3.pdf'));

form.submit('https://www.hypdf.com/pdfunite', function(err, res) {
    // res – response object (http.IncomingMessage)
    console.log(res.statusCode, res.resume());
});