private String rptFileLocation = "D:/Java Development/Projects/CustomerSubReport.rptdesign";
    private String pdfDownloadLocation = "E://logs/customer_subreport.pdf";
public void previewReportInPdf(String rptFileLocation, String pdfDownloadLocation) throws EngineException{
        IReportEngine engine = null;
        EngineConfig config = null;
        try{
            config = new EngineConfig();
            config.setLogConfig("E://logs", Level.FINEST);
            Platform.startup(config);
            IReportEngineFactory factory = 
                    (IReportEngineFactory)Platform.createFactoryObject(IReportEngineFactory.EXTENSION_REPORT_ENGINE_FACTORY);
            engine = factory.createReportEngine(config);
            IReportRunnable design = null;
            design = engine.openReportDesign(rptFileLocation);
            IRunAndRenderTask task = engine.createRunAndRenderTask(design);
            PDFRenderOption options = new PDFRenderOption();
            options.setOutputFileName(pdfDownloadLocation);
            options.setOutputFormat("pdf");
            task.setRenderOption(options);
            task.run();
            task.close();
            engine.destroy();
        }
        catch(Exception ex){
            ex.printStackTrace();
        }
        finally{
            Platform.shutdown();
        }
    }

<h:commandButton value="Preview Report(PDF)" action="#{birtBean.previewReportInPdf()}" ></h:commandButton>