How to collate multiple jrxml jasper reports into a one single pdf output file

Page numbers without itext...

private void drawPageNumbers(List<JasperPrint> listJasperPrint, int totalPages) throws JRException {

    int pageCount = 0;
    int posY = 0;
    int posX = 0;

    for (JasperPrint jasperPrint : listJasperPrint) {

        if (jasperPrint.getOrientation() == JRReport.ORIENTATION_PORTRAIT) {
            posY = 805;
            posX = 472;
        }

        if (jasperPrint.getOrientation() == JRReport.ORIENTATION_LANDSCAPE) {
            posY = 558;
            posX = 717;
        }

        for (Object obj : jasperPrint.getPages()) {

            pageCount++;
            JRPrintPage page = (JRPrintPage) obj;

            JRPrintText textTotalPages = new JRTemplatePrintText(new JRTemplateText(
                    jasperPrint.getOrigins()[0], jasperPrint.getDefaultStyleProvider()));
            textTotalPages.setX(posX + 54);
            textTotalPages.setY(posY);
            textTotalPages.setWidth(40);
            textTotalPages.setHeight(16);
            textTotalPages.setText(" " + totalPages);
            page.addElement(textTotalPages);

            JRPrintText textPageNumber = new JRTemplatePrintText(new JRTemplateText(
                    jasperPrint.getOrigins()[0], jasperPrint.getDefaultStyleProvider()));
            textPageNumber.setX(posX);
            textPageNumber.setY(posY);
            textPageNumber.setWidth(80);
            textPageNumber.setHeight(16);
            textPageNumber.setText("Página " + pageCount + " de");
            page.addElement(textPageNumber);
        }
    }

    return;
}

This answer is to help users with JASPER REPORT VERSION >5.6 (latest versions), hence remove the deprecated code.

Since jasper-report 5.6 JRPdfExporterParameter.JASPER_PRINT_LIST is deprecated the current code of Wojtek Owczarczyk answer is:

List<JasperPrint> jpList = new ArrayList<>();
//add your JasperPrint's from loading jrprint or more 
//commonly filling report with JasperFillManager.fillReport 

JRPdfExporter exporter = new JRPdfExporter();
exporter.setExporterInput(SimpleExporterInput.getInstance(jpList)); //Set as export input
exporter.setExporterOutput(new SimpleOutputStreamExporterOutput(stream)); //Set output stream
SimplePdfExporterConfiguration configuration = new SimplePdfExporterConfiguration();
//set your configuration
exporter.setConfiguration(configuration);
exporter.exportReport();

You can take advantage of exporting the whole jasperprint list:

List jpList = new ArrayList();
jpList.add(JRLoader.loadObjectFromFile("build/reports/Report1.jrprint")); 
...
JRExporter exporter = new JRPdfExporter(); 
exporter.setParameter(JRPdfExporterParameter.JASPER_PRINT_LIST, jpList); 
exporter.setParameter(JRPdfExporterParameter.OUTPUT_STREAM, stream); 
exporter.exportReport();