如何将生成的pdf文件上传到codeigniter中的文件夹？

Question

//控制器

function order_prescription()
{
     $this->load->model('prescription_model');
     $pre_master_id = $this->prescription_model->generate_prescription();
     $this->pdf($pre_master_id);
}
function pdf($pre_master_id)
{
    $this->load->helper('pdf_helper');
    /*$ViewData['tittle'] = 'Generate Prescription';
    $ViewData['breadcrumb'] = 'True';
    $ViewData['meta'] = 'Healthcare, Healthrepublic, Drugs';
    $ViewData['result'] =  $this->prescription_model->get_pescription_details($pre_master_id);*/
    $ViewData = array(
        'tittle' => 'mydashboard',
        'breadcrumb'=> 'True',
        'result'=>'',
        'meta' => 'Healthcare, Healthrepublic, Drugs',
        'result' => $this->prescription_model->get_pescription_details($pre_master_id)
                        );  
    $this->load->view('pdfreport', $ViewData);
    $this->index();
}

//生成pdf文件，但我想将此文件存储在一个文件夹中，所以给我一些建议