<apex:inputField id="IsPaidPd" value="{!Consultant_Request__c.Is_Paid_PD__c}" required="false">
                 <apex:actionSupport event="onchange" rerender="PurchaseOrderNumber_Wrapper" />
            </apex:inputField>

            <apex:pageBlockSectionItem >
            <apex:outputPanel id="PurchaseOrderNumber_Wrapper">
            <apex:outputLabel value="Purchase Order Number" for="PurchaseOrderNumber" rendered="{!Consultant_Request__c.Is_Paid_PD__c}"/>
                  <apex:inputField id="PurchaseOrderNumber" value="{!Consultant_Request__c.Purchase_Order_Number__c}" required="false" rendered="{!Consultant_Request__c.Is_Paid_PD__c}"/>
            </apex:outputPanel>
            </apex:pageBlockSectionItem>