index.js

var req = require('request');

req = req.defaults({
	agent: false
});

module.exports = function(buf, callback) {
	var r = req.post('http://mirror1.convertonlinefree.com', {
		encoding: null,
		headers: {
			'User-Agent': 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/37.0.2062.94 Safari/537.36'
		}
	}, function(err, res) {
		if (err) return callback(err);
		callback(null, res.body);
	});

	var form = r.form();
	form.append('__EVENTTARGET', '');
	form.append('__EVENTARGUMENT', '');
	form.append('__VIEWSTATE', '');
	form.append('ctl00$MainContent$fu', buf, {
		filename: 'output.docx',
		contentType: 'application/vnd.openxmlformats-officedocument.wordprocessingml.document'
	});

	form.append('ctl00$MainContent$btnConvert', 'Convert');
	form.append('ctl00$MainContent$fuZip', '');
};


if (require.main !== module) return;

var fs = require('fs');

module.exports(fs.readFileSync('test.docx'), function(err, data) {
	fs.writeFileSync('test.pdf', data);
});