How can send a visualforcepdf page as attachment while insert / update a record?

I have a requrement to send a visualforce PDF as an email attachemnt on insert a record A.

I am getting the error : " Update failed. First exception on row 0 with id a8d60000000TNzVAAW; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, IDS_SOShipperTrigger: execution of AfterUpdate caused by: System.VisualforceException: Getting content from within triggers is currently not supported".

My code is given below :

trigger IDS_SOShipperTrigger on rstk__soship__c (before insert, before update, after insert, after update) {
    
    Set<Id> shipperIds = new Set<Id>();
    Set<Id> vendorShippedShipperIds = new Set<Id>();
    Map<Id, String> shipperVendorEmails = new Map<Id, String>();
    System.debug('********************777');
    if(Master_Switches__c.getInstance('00e600000017Rhf').Triggers_On__c) {
        System.debug('********************666');
        if(Trigger.isBefore) {
            for(rstk__soship__c shipper : Trigger.New) {
                if(shipper.rstk__soship_closed__c == True
                   && Trigger.oldMap.get(shipper.Id).rstk__soship_closed__c == False) {
                    shipper.Shipment_Date__c = System.Today();
                }
            }
        }        
        else if(Trigger.isAfter) {
            for(rstk__soship__c shipper : Trigger.New) {
                if(shipper.rstk__soship_closed__c == True
                   && Trigger.oldMap.get(shipper.Id).rstk__soship_closed__c == False) {
                    shipperIds.add(shipper.Id);
                }
            }
            //System.debug('********************555');      
            if(shipperIds.size() > 0) {
                //Check whether shipment is sent by a vendor or not.
                for(rstk__sydata__c sydata : [SELECT Id, rstk__sydata_soship__c
                                              FROM rstk__sydata__c
                                              WHERE rstk__sydata_soship__c IN :shipperIds]) {
                    vendorShippedShipperIds.add(sydata.rstk__sydata_soship__c);   
                }
                System.debug('********************444');
                if(vendorShippedShipperIds.size() > 0) {
                    
                    for(rstk__soshipline__c shipperLine : [SELECT Id, rstk__soshipline_shipper__c,
                                                                  rstk__soshipline_line__r.PO_Line__r.rstk__poline_ordno__r.rstk__pohdr_vendno__r.Auto_Packing_List_Email__c
                                                           FROM rstk__soshipline__c
                                                           WHERE rstk__soshipline_shipper__c IN :vendorShippedShipperIds]) {
                        if(shipperLine.rstk__soshipline_line__r.PO_Line__r.rstk__poline_ordno__r.rstk__pohdr_vendno__r.Auto_Packing_List_Email__c != null) {
                            shipperVendorEmails.put(shipperLine.rstk__soshipline_shipper__c,
                                    shipperLine.rstk__soshipline_line__r.PO_Line__r.rstk__poline_ordno__r.rstk__pohdr_vendno__r.Auto_Packing_List_Email__c);
                        }
                    }
                    System.debug('********************333');
                    if(shipperVendorEmails.size() > 0) {
                        //Send emails as pdf attachment
                        Boolean successSend = True;
                        String emailResult = '';
                        List<String> selContactEmails = new List<String>();
                        List<Messaging.SingleEmailMessage> emailList = new List<Messaging.SingleEmailMessage>();
                        transient Messaging.EmailFileAttachment [] pdfAttachmentList = new List<Messaging.EmailFileAttachment> () ;
                        PageReference invoicePage = null;
                        invoicePage = Page.IDS_PackingSlip;
                        selContactEmails.add('jyothy@qburst.com');
                        System.debug('********************222');
                        for(String shipperIdValue : shipperVendorEmails.keySet()){                         
                            invoicePage.getParameters().put('Id', shipperIdValue);
                            invoicePage.setRedirect(true);   
                            Blob invoiceContent = !Test.isRunningTest() ? invoicePage.getContent() : Blob.ValueOf('dummy text');
                            Messaging.EmailFileAttachment pdfAttachment = new Messaging.EmailFileAttachment();
                            pdfAttachment.setFileName('Packing Slip ' + ' - ' + shipperIdValue + '.pdf');
                            pdfAttachment.setBody(invoiceContent);
                            pdfAttachment.setContentType('application/pdf');
                            pdfAttachmentList.add(pdfAttachment);  
                                              
                            Messaging.SingleEmailMessage email = new Messaging.SingleEmailMessage();  
                            selContactEmails.add(shipperVendorEmails.get(shipperIdValue));
                            email.setToAddresses(selContactEmails);
                            email.setSubject('Packing Slip '+ ' - ' + shipperIdValue );
                            email.setPlainTextBody(Label.Invoice_Email_Body);
                            if(pdfAttachmentList != null) {
                                email.setFileAttachments(pdfAttachmentList) ;
                            }
                            emailList.add(email);
                            System.debug('********************111');
                        }
                        if(emailList.size() > 0){
                            Messaging.SendEmailResult[] sendResults = Messaging.sendEmail(emailList);
                            //Get error message if any
                            for(Messaging.SendEmailResult result : sendResults) {
                                if(!result.isSuccess()) {
                                    emailResult = result.getErrors()[0].getMessage();
                                }
                            }
                            if(String.isBlank(emailResult)){
                                emailResult = Label.IDS_Invoice_Sent_Successful_Message;
                                successSend = false;
                            }
                        }    
                    }
                }
            }
        }
    }
}

Any help? Thanks in advance :)