I’ve done a project using Prince recently, which is a very powerful and flexible tool (although quite expensive).
A few weeks ago I discovered WeasyPrint, which is actually not a service but an open source tool that seems to be quite similar to Prince in that it supports CSS pretty well. I have not tested it, but it looks very promising.
@kmajort All these tools generally support writing the PDF to disk, so you should be able to save the file into the content directory or (to send it as PDF) into a temporary directory and then call the $kirby->email() method on it.