public PageReference generatePdf() {
       //PageReference pdf = new PageReference('/apex/yourPageName');
       //Page.yourPageName;
       PageReference targetPage = Page.yourPageName;
       targetPage.getParameters().put('id',objOpp.Id);
       //pdf.setRedirect(true);
       system.debug('<<<<<<<pdf : '+targetPage);

       Attachment attach = new Attachment();
       Blob targetBody;
    //   try {
           targetBody = targetPage.getContentAsPDF();
    //   } catch ( VisualforceException exp ) {
        //    targetBody = Blob.valueof('There was an error.');
    //   }
       attach.Body = targetBody;
       attach.ContentType = 'application/pdf';
       attach.Name = 'Final quote- '+objOpp.Name +' '+String.valueof(Datetime.now())+'.pdf';
       attach.IsPrivate = false;
       attach.ParentId = objOpp.Id;
       insert attach;
       return null;
    }