PhantomJS: pipe input
You can do what you're looking for very simply (it's just not really documented) directly in PhantomJS.
var page = require('webpage').create(), fs = require('fs');page.viewportSize = { width: 600, height: 600 };page.paperSize = { format: 'Letter', orientation: 'portrait', margin: '1cm' };page.content = fs.read('/dev/stdin');window.setTimeout(function() { page.render('/dev/stdout', { format: 'pdf' }); phantom.exit();}, 1);(May need to increase the timeout if you have images that need loading, etc.)
HTML comes in stdin, PDF binary goes out stdout. You can test it like:
echo "<b>test</b>" | phantomjs makepdf.js > test.pdf && open test.pdf