This article is a good read discussing various options, my preference is sagas https://medium.com/@jeffbski/where-do-i-put-my-business-logic-in-a-react-redux-application-9253ef91ce1#.g249gmnvz

Sometime ago I generated PDFs on server using ColdFusion which had some nice PDF generation functions/tags. Since moving on from ColdFusion to NodeJS I've used DocRaptor which has been excellent: https://docraptor.com/ The main downside is pricing approach which is tiered and not simply credit based (like Postmark) and no easy way to set an account to idle. These may not be big issues to you. I have one application that is highly seasonal so really need pricing to handle high peaks and then many months with zero usage. DocRaptor has API samples for PHP, Ruby, NodeJS and others. It can also handle XLS. All said the ease of use and not having to worry about uptime is well worth it for me.