Sorry for not answering.
Yes, your approach is the correct one. In case you want an example of a customized pdf, here is one
https://github.com/asciidoctor/asciidoctor-maven-examples/tree/master/asciidoctor-pdf-with-theme-example.
And I assume you already found the docs
https://github.com/asciidoctor/asciidoctor-pdf/blob/master/docs/theming-guide.adocIf this for a rest service, you could send the style as an optional parameter so that clients can tune the end result.
Btw, just in case, my advice is to use POST multipart. You can send the different input files as different parts and even send other parameters (e.g. use a part to send a JSON sin attributes). A nice extra is that you can build a plain HTML form to invoke you service without any extra code.