You could bake this into the REST API via the "Accept:" header. The default when not specified is whatever you set, but the caller can specify "Accept: image/pn

You could bake this into the REST API via the "Accept:" header. The default when not specified is whatever you set, but the caller can specify "Accept: image/png" to get a PNG image, "Accept: application/pdf" for PDF, and so on for whatever other types of output you support. The advantage of PDF (or SVG even) is that, if it was constructed using text/vectors, the output resolution is limited only by the capabilities of the printer.