Hi Oscar,
Try the below code:
import com.sap.eso.api.doccommon.ont.types.BigTextValueIBeanIfc;
import com.sap.odp.api.common.types.TypeFactory;
bigTextObj=TypeFactory.createBigText("Hello world").getTextString(session);
doc.getExtensionField(RFID).set(bigTextObj);
This will solve your problem!!
Regards,
Vignesh