@Async
public synchronized void carregarApresentacoesParaExportar() {
    try {
        ...............
        for(int i = 0 ; i < qtdePages; i++){
            if(isBaixado){
                percent = 0;
                qtdePages=0;
                break;
            } else {
                exportar.configurarValores(transacoes.getContent());
                if(transacoes.hasNext()){
                        transacoes = getService(enumType).buscarPaginado(getFiltro(), new PageRequest(transacoes.nextPageable().getPageNumber(), MAX_EXPORTAR));
                }
                percent = ((i * 100) / qtdePages);
            }
        }
        ...............
    } catch (Exception e) {
        LOG.error(e, e);
        geraMsgErro("txt.erro.download.excel");
        FileUtils.deleteQuietly(arquivo);
    }
}

<p:progressBar widgetVar="pbAjax" ajax="true" value="#{notificacaoMB.percent}" labelTemplate="{value}%"
                                                styleClass="animated" global="false">
                                                <p:ajax event="complete" listener="#{notificacaoMB.onComplete}" />
                                            </p:progressBar>

                                            Outra ProgressBar

                                            <div class="progress progress-striped active">
                                                <div style="width: #{notificacaoMB.percent}%" id="progress" aria-valuemax="100" aria-valuemin="0" aria-valuenow="#{notificacaoMB.percent}" role="progressbar" class="progress-bar progress-bar-success">
                                                    <span >#{notificacaoMB.percent}%</span>
                                                </div>
                                            </div>

<h:commandButton title="#{messages['label.iniciar']}"
                                                value="#{messages['label.iniciar']}"
                                                actionListener="#{notificacaoMB.carregarApresentacoesParaExportar()}"
                                                onclick="PF('pbAjax').start();PF('startButton').disable();"
                                                widgetVar="startButton"
                                                rendered="#{notificacaoMB.possoIniciar()}"
                                                styleClass="btn btn-primary" />