How to serve files/PDF files the reactive way in spring

The easiest way to achieve that would be with a Resource.

@GetMapping(path = "/pdf", produces = "application/pdf")
ResponseEntity<Resource> getPDF() {
  Resource pdfFile = ...;
  HttpHeaders headers = new HttpHeaders();
  headers.setContentDispositionFormData(file.getName(), file.getName());
  return ResponseEntity

Note that DataBufferUtils has some useful methods there that convert an InputStream to a Flux<DataBuffer>, like DataBufferUtils#read(). But dealing with a Resource is still superior.