from qgis.core import (QgsProject, QgsLayoutExporter, QgsApplication)


QgsApplication.setPrefixPath("C:/OSGeo4W64/apps/qgis", True)

gui_flag = False
app = QgsApplication([], gui_flag)

app.initQgis()

project_path = '//192.168.172.131/38262/I/2015PN37-T3C/19-avoisinants/SIG/L15 T3_MAP_v10_20190315_QGIS3.qgz'

project_instance = QgsProject.instance()
project_instance.setFileName(project_path)
project_instance.read()

manager = QgsProject.instance().layoutManager()
tun_list = ["TR1","TR2"]
for tun in tun_list:
    layout = manager.layoutByName(tun)

    exporter = QgsLayoutExporter(layout)
    exporter.exportToPdf('e:\\Users\\louis.monnier\\Desktop\\L15-T3C\\Suivi ouvrages\\TR1\\Données tunnelier\\qgis_%s.pdf'%(tun), QgsLayoutExporter.PdfExportSettings())

app.exitQgis()