I always found SVG (plus the Inkscape CLI) to be the best interface for generating PDF documents.

I'm sure there are arguments in favour of libraries like this such as performance or better control but when all you need is to generate a simple invoice with a couple of dynamic fields, just use Inkscape to draw your template and your favourite XML library to put values in it in your code. When you need the PDF, use inkscape --export-pdf to get your PDF document.

SVG also makes it quite easy to show an in-browser preview of the generated document before performing the actual conversion, avoiding a costly PDF generation most of the time.