<apex:page controller="BDPaginationController" >
    <apex:form >
        <apex:actionFunction name="refreshPageSize" action="{!refreshPageSize}" status="fetchStatus" reRender="pbId"/>
        <apex:pageBlock id="pbId" title="Billing Details of this Month">
            <apex:pageBlockSection collapsible="false" columns="1">
                <apex:pageBlockTable value="{!BD}" var="b">
                    <apex:column >
                   <apex:outputLink title="" value="/{!b.Id}/e?retURL=/apex/{!$CurrentPage.Name}" style="font-weight:bold">Edit</apex:outputLink>&nbsp;|&nbsp;
					<a href="javascript&colon;if (window.confirm('Are you sure?')) DeleteBD'{!b.Id}');" style="font-weight:bold">Del</a>
					</apex:column>
					<apex:column value="{!b.Name}"/>
					<apex:column value="{!b.Billing_Type__c}"/>
					<apex:column value="{!b.Billing_Period__c}"/>    
					<apex:column value="{!b.Monthly_Forecast__c}"/>
					<apex:column value="{!b.End_of_Billing__c}"/>
					<apex:column value="{!b.Amount__c}"/>
					<apex:column value="{!b.Billing_Status__c}"/>
                </apex:pageBlockTable>
                 
                <apex:panelGrid columns="8"> 
                 
                <apex:selectList value="{!size}" multiselect="false" size="1" onchange="refreshPageSize();">
                    <apex:selectOptions value="{!paginationSizeOptions}"/>
                </apex:selectList>
                 
                <apex:commandButton status="fetchStatus" reRender="pbId" value="First" action="{!setCon.first}" disabled="{!!setCon.hasPrevious}" title="First Page"/> 
  
                <apex:commandButton status="fetchStatus" reRender="pbId" value="Previous" action="{!setCon.previous}" disabled="{!!setCon.hasPrevious}" title="Previous Page"/> 
  
                <apex:commandButton status="fetchStatus" reRender="pbId" value="Next" action="{!setCon.next}" disabled="{!!setCon.hasNext}" title="Next Page"/> 
  
                <apex:commandButton status="fetchStatus" reRender="pbId" value="Last" action="{!setCon.last}" disabled="{!!setCon.hasNext}" title="Last Page"/> 
  
                <apex:outputText >{!(setCon.pageNumber * size)+1-size}-{!IF((setCon.pageNumber * size)>noOfRecords, noOfRecords,
                     (setCon.pageNumber * size))} of {!noOfRecords}
                </apex:outputText> 
                       
                <apex:outputPanel >                      
                    <apex:actionStatus id="fetchStatus" >
                        <apex:facet name="start" >
                          <img src="/img/loading.gif" />                    
                        </apex:facet>
                    </apex:actionStatus>
                </apex:outputPanel> 
  
            </apex:panelGrid>  
            </apex:pageBlockSection>
            <apex:pageBlockButtons >
 				<apex:commandButton value="Move to next month" action="!next"/>   
			</apex:pageBlockButtons>
        </apex:pageBlock>
        
        //aggiungo
         <apex:commandLink action="{!deleteBD}" onclick="if(!confirm('Are you sure?')) return false;">Del
    <apex:param value="{!b.Id}" name="idToDel" assignTo="{!BdId}"/>
</apex:commandLink>

        
        
    </apex:form>
</apex:page>

public class BDPaginationController{
    Public Integer size{get;set;} 
    Public Integer noOfRecords{get; set;} 
    public List<SelectOption> paginationSizeOptions{get;set;}
         
    public BDPaginationController(){
        size=10;
        paginationSizeOptions = new List<SelectOption>();
        paginationSizeOptions.add(new SelectOption('5','5'));
        paginationSizeOptions.add(new SelectOption('10','10'));
        paginationSizeOptions.add(new SelectOption('20','20'));
        paginationSizeOptions.add(new SelectOption('50','50'));
        paginationSizeOptions.add(new SelectOption('100','100'));
    }
     
    public ApexPages.StandardSetController setCon {
        get {
            if(setCon == null) {                
                setCon = new ApexPages.StandardSetController(Database.getQueryLocator(
                      [Select Id,Name,Billing_Type__c, Billing_Period__c , Monthly_Forecast__c,End_of_Billing__c, Amount__c, Billing_Status__c   from Billing_Detail__c Where Monthly_Forecast__c=THIS_MONTH ]));
                setCon.setPageSize(size);  
                noOfRecords = setCon.getResultSize();
            }            
            return setCon;
        }
        set;
    }
     
    //Changes the size of pagination
    public PageReference refreshPageSize() {
         setCon.setPageSize(size);
         return null;
    }
 
    // Initialize setCon and return a list of record    
     
    public List<Billing_Detail__c> getBD() {
         return (List<Billing_Detail__c>) setCon.getRecords();
    }
    

	public List<Billing_Detail__c> listBD {get;set;}
	public String SelectedBdId {get;set;}
			
	public void loadData() {
		listBD = [Select Id,Name,Billing_Type__c, Billing_Period__c , Monthly_Forecast__c,End_of_Billing__c, Amount__c, Billing_Status__c   from Billing_Detail__c Where Monthly_Forecast__c=THIS_MONTH ];
	}

//to delete
	public void deleteBD(){
       listBD = [Select Id,Name,Billing_Type__c, Billing_Period__c , Monthly_Forecast__c,End_of_Billing__c, Amount__c, Billing_Status__c   from Billing_Detail__c Where Monthly_Forecast__c=THIS_MONTH AND id =:BdId];
      if(listBD.size() > 0 || listBD[0].Id != ''){
      delete listBD;
      }
      loadData();
  
   }
  
   
}

• vf page: Unknown method 'BDPaginationController.deleteBD()' (supposed to the other )
• BDpaginationController: Variable does not exist: BdId