Below is my complete process, modified few things:

set up remote site Setup->Security Controle->Remote Site setting. click on New Remote Site

Enter the detailes as in the image and "remote site url" is your ord url

Schedule class:-
global class ScheduleFinacialNotification implements Schedulable {
global string sendSessionId;

global ScheduleFinacialNotification(String sessionId){
  this.sendSessionId = sessionId;
}
global void execute(SchedulableContext SC) {
     BatchSendFinacialNotification batchApex=new BatchSendFinacialNotification();
     batchApex.sessionId = this.sendSessionId;
     Database.executeBatch(batchApex,1);  }}

Batch class:-

global class BatchSendFinacialNotification implements Database.Batchable<sObject>,Database.Stateful,Database.AllowsCallouts {
public String query;
global string sessionId;
global BatchSendFinacialNotification(){
  this.sessionId  = sessionId;     
        query    = 'Select Id,Name,Project__r.Go_Live_Date__c from Service_Project_Junction__c ';
    }
    global Database.QueryLocator start(Database.BatchableContext BC){
        System.debug('=========='+Database.getQueryLocator(query));
        return Database.getQueryLocator(query);
    }
  
global void execute(Database.BatchableContext BC, List<Service_Project_Junction__c> scope) {
  //List<Service_Project_Junction__c> pojJunRecList = [Select Id,Name,Project__c,Opportunity_Deal__c,Project__r.Id,Project__r.Go_Live_Date__c,Product_Opportunity_Junction__r.Service__r.Service_Type__c,Product_Opportunity_Junction__r.Service__r.Product_Price__c,Product_Opportunity_Junction__r.No_Of_Months__c,Product_Opportunity_Junction__r.Total_Price__c,Product_Opportunity_Junction__r.No_Of_Listings__c from Service_Project_Junction__c where Opportunity_Deal__c != null AND Project__r.Go_Live_Date__c != null];
 
  for(Service_Project_Junction__c proRec: scope){
 
   Integer duration = integer.valueOf(proRec.Product_Opportunity_Junction__r.No_Of_Months__c);
   Integer totMonthTillToday = proRec.Project__r.Go_Live_Date__c.monthsBetween(System.today());
   System.debug('Test in for loop==>'+proRec);
   if(totMonthTillToday <= duration && proRec.Project__r.Go_Live_Date__c.Day() == Date.today().day()){
    System.debug(sessionId+'Test in if loop==>'+this.sessionId);
    ServiceInvoiceRedirectController.attachPdfToRecord( proRec.Project__r.Id, this.sessionId);
    System.debug('Test in if loop after condiyion==>');
   }
  }
}

global void finish(Database.BatchableContext BC){     
    }
  
}

Call of REST Api class:-

@RestResource(urlMapping='/AttachPDF/*')
global class ServiceInvoiceRedirectController
{
@HttpGet
        global static void AttachPDFtoRecordREST()
{
       
            RestRequest req = RestContext.request;
            id recordId = req.requestURI.substring(req.requestURI.lastIndexOf('/')+1);

            PageReference pdfPage = new PageReference('/apex/ServiceInvoicePDF?Id='+recordId);
            pdfPage.getParameters().put('id',recordId);
            Blob pdf = pdfPage.getContentAsPdf(); //!Test.isRunningTest() ? pdfPage.getContentAsPdf() : Blob.ValueOf('dummy text');
          
            Project__c prjRec = [Select Id,Name,Builder_Name__c from Project__c where Id =: recordId];
            //Insert Invoice record for related project
         Invoice__c invoRec = new Invoice__c();
         invoRec.RecordTypeId = Constants_PicklistVariables.serInvoice;
         invoRec.Project__c = recordId;
         invoRec.Account__c = prjRec.Builder_Name__c;
         try{
         insert invoRec;
         }catch(Exception e){
             System.debug('Invoice=Exception-->'+e);
         }
         System.debug('Invoice=invoRec.id-->'+invoRec);
       
         //Insert Attacment to the invoice      
         Attachment a = New Attachment();
         a.body = pdf;
         a.parentID = invoRec.id;
         a.ContentType  = 'application/pdf';
         a.Name = 'SER-INV-'+invoRec.id+'.pdf';
         try{
         insert a;
         }catch(Exception e){
             System.debug('Attachment=Exception-->'+e);
         }
       
         //Create the email attachment
      Messaging.EmailFileAttachment efa = new Messaging.EmailFileAttachment();
      efa.setFileName('pdfName.pdf');//Pleace ad pdf name from contact name project and oppty concatinate
      efa.setContentType('application/pdf');
      efa.setBody(pdf);
    
      // Create the Singal Email Message
      Messaging.SingleEmailMessage email = new Messaging.SingleEmailMessage();
      email.setSubject('Financial Notification');
      email.setToAddresses(new String[] { 'dagny.fernandes@extentor.com' });//add account email id
      email.setPlainTextBody( 'Please Find Attachment your financial report Attached with this email.');
      email.setFileAttachments(new Messaging.EmailFileAttachment[] {efa});
    
      // Sends the email
      Messaging.SendEmailResult [] r = Messaging.sendEmail(new Messaging.SingleEmailMessage[] {email});
          
        }

        // call this method from your Batch Apex
        global static void attachPdfToRecord( Id recordId, String sessionId )
        {
         System.debug('recordId==>'+recordId);
         System.debug('sessionId==>'+sessionId);
         System.debug('Iam in REST API Call method');
           // String addr = 'https://cs5.salesforce.com/services/apexrest/AttachPDF/' + recordId;
            HttpRequest req = new HttpRequest();
            req.setEndpoint('https://cs5.salesforce.com/services/apexrest/AttachPDF/a04O000000EHJN6IAP');
            req.setMethod('GET');
            req.setHeader('Authorization', 'OAuth ' + sessionId);

            Http http = new Http();
            HttpResponse response = http.send(req);  
        }
      
}


Schedule class from Developer Console in Anonymous Window:-

Datetime sysTime = System.now();
sysTime = sysTime.addSeconds(5);
String chron_exp =' '+sysTime.second() + ' ' + sysTime.minute() + ' ' + sysTime.hour() + ' ' + sysTime.day() + ' ' + sysTime.month() + ' ? ' + sysTime.year();
System.schedule('Job1',chron_exp , new ScheduleFinacialNotification(UserInfo.getSessionId()));