Thanks to Alex K I found the answer:
according to JRPdfExporterParameter.html#PDF_JAVASCRIPT
you can use PDF_JAVASCRIPT
property to add javascript to the pdf when you generate it.
so I added
JRPdfExporter exporter = new JRPdfExporter();
exporter.setParameter(JRPdfExporterParameter.PDF_JAVASCRIPT, "this.print();");
when exporting PDF and it worked