Test class:-

@istest
public class AddAttachmentOnQuotes_Test{
     Static testMethod Void testMethod2(){
           Test.starttest();

             Account acc = new Account();
             acc.Name = 'Test Account';
             acc.Website ='www.test.com';
             acc.Type='Banking';
             insert acc;
             
             opportunity opp = new opportunity();
             opp.name ='Test Opp';
             opp.stagename='Closed Won';
             opp.Type= 'New Business';
             opp.CloseDate=system.today().addmonths(2);
             opp.Upsell_Potential__c ='No';
             opp.RenewalDate__c = system.today();

             opp.accountid=acc.id;
            insert opp;  
            
                SBQQ__Quote__c oldquote = new SBQQ__Quote__c();
                oldquote.SBQQ__Primary__c = true;
                oldquote.SBQQ__Opportunity2__c = opp.id;
                oldquote.SBQQ__StartDate__c = system.today();
                oldquote.SBQQ__EndDate__c= system.today().addmonths(3);
                oldquote.SBQQ__SubscriptionTerm__c=1;
            
                oldquote.Status_Of_Approval__c='Pending';
                oldquote.Document_Template__c= 'Auto Renew';

                oldquote.SBQQ__PaymentTerms__c= 'monthly';
               // oldquote.Contract_End_Date__c = system.today().addmonths(3);
                insert oldquote;
                
                oldquote.Document_Template__c= 'Auto Renew 3';
                oldquote.SBQQ__PaymentTerms__c= 'monthly';
                update oldquote;
                
                 oldquote.Document_Template__c= 'Basic Template';
                oldquote.SBQQ__PaymentTerms__c= 'monthly';
                update oldquote;
            
            
             
         }   
            
          
}



Apex handler:-

Public class AddAttachmentOnQuotes {
    public static void createAttachment(List<SBQQ__Quote__c> quotes){
        List<Attachment> attls = [select id, Name, body from Attachment where name like '%Recurring Payment Agreement Form_2%' limit 1];
        List<Attachment> newAttls = new List<Attachment>();
        
        if(attls.size() > 0) { 
            for(SBQQ__Quote__c q1 :quotes) {
               Attachment newFile = attls[0].clone();
               newFile.ParentId = q1.id;
               newAttls.add(newFile);
                
            }
        }
        if(newAttls.size() > 0) {
            insert newAttls;
        }
        
        List<SBQQ__RelatedContent__c> relConls = new List<SBQQ__RelatedContent__c>();
        for(Attachment newAtt :newAttls) {
            SBQQ__RelatedContent__c relCon = new SBQQ__RelatedContent__c();
            relCon.Name = newAtt.Name;
            relCon.SBQQ__Quote__c = newAtt.parentId;
            relCon.SBQQ__ExternalId__c = newAtt.Id;
            relCon.SBQQ__Required__c = true;
            relConls.add(relCon); 
        }
        if(relConls.size()>0) {
            insert relConls;
        }
    
    }
}

Apex trigger :-

trigger QuoteTrigger_PayTerms_GenerateDoc on SBQQ__Quote__c (after insert, after update) {


        if(Trigger.Isinsert) {
            pt.Documenttemplate(Trigger.new);
            AddAttachmentOnQuotes.createAttachment(Trigger.new);
        }
       
        // for quote creation...
        if(Trigger.IsUpdate) {
            List<SBQQ__Quote__c> quotels = new List<SBQQ__Quote__c>();
            Set<Id> quoteIds = new Set<Id>();
            
            for(SBQQ__Quote__c qt : trigger.new) {
                quoteIds.add(qt.id);
            }
            set<Id> existingQuoteIds = new Set<Id>();
            List<Attachment> attls = [select id, Name,parentId from Attachment where parentId = :quoteIds];
            for(Attachment att :attls) {
                existingQuoteIds.add(att.id);
            }
            for(SBQQ__Quote__c qt : trigger.new) {
            
                if((qt.Document_Template__c == 'Auto Renew' && ( qt.SBQQ__PaymentTerms__c=='Monthly' ||  qt.SBQQ__PaymentTerms__c=='Quarterly'))
                
                    ||(qt.Document_Template__c=='Auto Renew 3' && ( qt.SBQQ__PaymentTerms__c=='Quarterly' ||  qt.SBQQ__PaymentTerms__c=='Monthly'))
                    
                    ||(qt.Document_Template__c=='Basic Template' && ( qt.SBQQ__PaymentTerms__c=='Quarterly' ||  qt.SBQQ__PaymentTerms__c=='Monthly'))
                    
                    && qt.Document_Template__c != trigger.oldMap.get(qt.id).Document_Template__c
                    
                    && !existingQuoteIds.contains(qt.Id)) {
                    
                        quotels.add(qt);
                }
            }
            if(quotels.size() > 0) {
               AddAttachmentOnQuotes.createAttachment(Trigger.new); 
            }    
        }
            
}