Thanks for your reply Ron, I am intializing each element of PDFClass.

Here I am posting entire code, pls have a look.

One more problem is I am getting an error like "Already Output".

<apex:page standardController="Order_Header__c" extensions="Order_PDF">
<apex:form >
<apex:pageBlock >
<apex:commandButton action="{!Pdf}" value="Get PDF"/>
<apex:repeat value="{!PDFList}" var="p">
<apex:pageblockSection title="{!p.ol.Name}" columns="1">
<apex:repeat value="{!p.APB}" var="a1">
<apex:pageBlock title="TEST"></apex:pageBlock>
<apex:outputText value="{!a1.Id}"/>
</apex:repeat>
<apex:repeat value="{!p.CD}" var="a2">
<apex:pageBlock title="TEST"></apex:pageBlock>
<apex:outputText value="{!a2.Name}"/>
</apex:repeat>
<apex:repeat value="{!p.CS}" var="a3">
<apex:pageBlock title="TEST"></apex:pageBlock>
<apex:outputText value="{!a3.Name}"/>
</apex:repeat>
<apex:repeat value="{!p.EI}" var="a4">
<apex:pageBlock title="TEST"></apex:pageBlock>
<apex:outputText value="{!a4.Name}"/>
</apex:repeat>
<apex:repeat value="{!p.Feature}" var="a5">
<apex:pageBlock title="TEST"></apex:pageBlock>
<apex:outputText value="{!a5.Name}"/>
</apex:repeat>
<apex:repeat value="{!p.NB}" var="a6">
<apex:pageBlock title="TEST"></apex:pageBlock>
<apex:outputText value="{!a6.Name}"/>
</apex:repeat>
<apex:repeat value="{!p.RRC}" var="a7">
<apex:pageBlock title="TEST"></apex:pageBlock>
<apex:outputText value="{!a7.Name}"/>
</apex:repeat>
<apex:repeat value="{!p.TD}" var="a8">
<apex:pageBlock title="TEST"></apex:pageBlock>
<apex:outputText value="{!a8.Name}"/>
</apex:repeat>
</apex:pageblockSection>
</apex:repeat>
</apex:pageBlock>
</apex:form>
</apex:page>

Public class Order_PDF
{
public Order_PDF(ApexPages.StandardController controller)
{
}

Billing_Account_Order_Use__c BAOU;
List<Order_Line_Item__c> orderLineItems = new List<Order_Line_Item__c>();
List<Access_Port_Backup__c> APBList = new List<Access_Port_Backup__c>();
List<Circuit_Details__c> CDList = new List<Circuit_Details__c>();
List<CoS_Segment__c> CSList = new List<CoS_Segment__c>();
List<EVC_Information__c> EIList = new List<EVC_Information__c>();
List<Features__c> FeatureList = new List<Features__c>();
List<Network_Based__c> NBList = new List<Network_Based__c>();
List<Router_RouterCards__c> RRCList = new List<Router_RouterCards__c>();
List<Technical_Details__c> TDList = new List<Technical_Details__c>();
List<Billing_Account_Order_Use__c> BAOUList = new List<Billing_Account_Order_Use__c>();
List<Id> idList = new List<Id>();

List<Access_Port_Backup__c> newAPBList = new List<Access_Port_Backup__c>();
List<Circuit_Details__c> newCDList = new List<Circuit_Details__c>();
List<CoS_Segment__c> newCSList = new List<CoS_Segment__c>();
List<EVC_Information__c> newEIList = new List<EVC_Information__c>();
List<Features__c> newFeatureList = new List<Features__c>();
List<Network_Based__c> newNBList = new List<Network_Based__c>();
List<Router_RouterCards__c> newRRCList = new List<Router_RouterCards__c>();
List<Technical_Details__c> newTDList = new List<Technical_Details__c>();
List<pdfclass> PDFList = new List<pdfclass>();

Public PageReference Pdf()
{
Id ordId = System.currentPageReference().getParameters().get('id');
//BAOU = [select Name,Billing_Account__c,Billing_Account__r.Name,Billing_Account__r.Billing_Account__c,Site_Name__r.Name,Order_Number__r.Name from Billing_Account_Order_Use__c where Order_Number__c=:ordId];
orderLineItems = [Select VLAN_DLCI__c, Access_Method__c, ATQ_Reference__c from Order_Line_Item__c where Order_Number__c = :ordId];
for(Order_Line_Item__c oli:orderLineItems)
idList.add(oli.id);

APBList = [Select Unit_Discount_Monthly_Rent__c, Backup_Access_Type__c, Access_Type__c from Access_Port_Backup__c where Order_Line_Item__c in :idList];

CDList = [Select Total_Monthly_Rental_Charge__c, Access_Type__c from Circuit_Details__c where Order_Line_Item__c in :idList];

CSList = [Select Unit_Discount_Monthly_Rent__c, Configuration_Type_BackUp__c from CoS_Segment__c where Order_Line_Item__c in :idList];
EIList = [Select VLAN_Identifier_on_which__c, Amortisation_Period__c from EVC_Information__c where Order_Line_Item__c in :idList];

FeatureList = [Select Unit_Discount_Monthly_Rent__c, CreatedById from Features__c where Order_Line_Item__c in :idList];

NBList = [Select Unit_Discount_Monthly_Rent__c, CreatedById from Network_Based__c where Order_Line_Item__c in :idList];
RRCList = [Select Unit_Discount_Monthly_Rental__c, Customer_Router__c from Router_RouterCards__c where Order_Line_Item__c in :idList];
TDList = [Select xDSL_line_type__c, UK_BT__c, Access_Resilience_Diversity_and_Separacy__c from Technical_Details__c where Order_Line_Item__c in :idList];

for(Order_Line_Item__c oli:orderLineItems)
{
for(Integer i=0; i<APBList.size(); i++)
if(APBList[i].Order_Line_Item__c == oli.id)
{
newAPBList.add(APBList[i]);
APBList.remove(i);
}
for(Integer i=0; i<CDList.size(); i++)
if(CDList[i].Order_Line_Item__c == oli.id)
{
newCDList.add(CDList[i]);
CDList.remove(i);
}
for(Integer i=0; i<CSList.size(); i++)
if(CSList[i].Order_Line_Item__c == oli.id)
{
newCSList.add(CSList[i]);
CSList.remove(i);
}
for(Integer i=0; i<EIList.size(); i++)
if(EIList[i].Order_Line_Item__c == oli.id)
{
newEIList.add(EIList[i]);
EIList.remove(i);
}
for(Integer i=0; i<FeatureList.size(); i++)
if(FeatureList[i].Order_Line_Item__c == oli.id)
{
newFeatureList.add(FeatureList[i]);
FeatureList.remove(i);
}
for(Integer i=0; i<NBList.size(); i++)
if(NBList[i].Order_Line_Item__c == oli.id)
{
newNBList.add(NBList[i]);
NBList.remove(i);
}
for(Integer i=0; i<RRCList.size(); i++)
if(RRCList[i].Order_Line_Item__c == oli.id)
{
newRRCList.add(RRCList[i]);
RRCList.remove(i);
}
for(Integer i=0; i<TDList.size(); i++)
if(TDList[i].Order_Line_Item__c == oli.id)
{
newTDList.add(TDList[i]);
TDList.remove(i);
}
PDFList.add(new pdfclass(oli, newAPBList, newCDList, newCSList, newEIList, newFeatureList, newNBList, newRRCList, newTDList));
newAPBList.clear();
newCDList.clear();
newCSList.clear();
newEIList.clear();
newFeatureList.clear();
newNBList.clear();
newRRCList.clear();
newTDList.clear();
}
return null;
}
Public List<pdfclass> getPDFList()
{
system.debug('PDFLIST::::::'+pdflist);
return PDFList;
}
Public class pdfclass
{
Public Order_Line_Item__c Ol{get; set;}
Public List<Access_Port_Backup__c> APB{get; set;}
Public List<Circuit_Details__c> CD{get; set;}
Public List<CoS_Segment__c> CS{get; set;}
Public List<EVC_Information__c> EI{get; set;}
Public List<Features__c> Feature{get; set;}
Public List<Network_Based__c> NB{get; set;}
Public List<Router_RouterCards__c> RRC{get; set;}
Public List<Technical_Details__c> TD{get; set;}
public pdfclass(Order_Line_Item__c O, List<Access_Port_Backup__c> A, List<Circuit_Details__c> C, List<Cos_Segment__c> Cos, List<EVC_Information__c> E, List<Features__c> F, List<Network_Based__c> N, List<Router_RouterCards__c> R, List<Technical_Details__c> T)
{
ol = o;
APB = a;
CD = c;
CS = cos;
EI = e;
Feature = f;
NB = n;
RRC = r;
TD = t;
}
}
}

 I feel like everything is correct, but I am missing some point, could you point me where I need to change.

Thanks in Advance.