The pdfRenderingService provided by the plugin allows you to call render and get back an OutputStream. Using that output stream you can write that to a file on your server. The documentation explains the basics of using the service.
Your code may look something like this:
new File("report.pdf").withOutputStream { outputStream ->
outputStream << pdfRenderingService.render(template: '/report/report', model: [serial: 12345])
}