Weird pipe behavior in node's child_process?

var spawn = require('child_process').spawn;

var fd_stdin = fs.openSync('lk.log', 'r');
spawn('node', ['monitor.js'], {
    stdio: [fd_stdin, 1, 2];
});

var spawn = require("child_process").spawn;

var ls = spawn("ls -a");
ls.stdout.pipe(process.stdout);
ls.stderr.pipe(process.stderr);
ls.on("exit", function(code) {
    process.exit(code);
});

 var args = ['-c', <the entire command you want to run as a string>];

S3Fs.readFile('./my-pdf-in-s3-bucket', (err, result) => {
  if (err) throw err; // Handle better
  var cp = child_process.spawn('pdftotext', [ '-', '-' ]);
  cp.stdout.pipe(process.stdout);
  cp.on('close', (code, signal) => {
    console.log(`pdftotext finished with status ${code}`);
  });
  cp.stdin.end(result);
});

var cp = child_process.spawn('pdftotext', [ '-', '-' ]);
var rs = S3Fs.createReadStream('./my-pdf-in-s3-bucket');
rs.on('error', (err) => {
  cp.kill();
});
cp.stdout.pipe(process.stdout);
cp.on('close', (code, signal) => {
  console.log(`pdftotext finished with status ${code}`);
});
rs.pipe(cp.stdin);