Document Studio is a new Google Sheets add-on that allows you to create certificates, personalized letters, purchase orders and other types of documents. This add-on allows you to take information from a spreadsheet and merge it into a template (Docs or Slides) that you have created. This is a fast and easy way to create customized and visually appealing documents without having to do much work.
The process is a little complicated because there are several steps involved, but the creator of the add-on made a wonderful video that walks you through the entire process.
Autocrat is another document merge tool. I would recommend giving both add-ons a try to see which one fits your particular needs.