Problem uploading PDF Files

[eluser]sherpa[/eluser]
Hi Everybody. I want to share my problem here. My problem is that i can't upload PDF files. I have created a function which meants to upload as many PDF files as i want passing file arrays and filepath arrays to the function as parameters. Everytime I try, it says The filetype you are attempting to upload is not allowed. However, i can upload JPG and GIF files with the same code changing extension without any errors. I don't know whats wrong with my following code. Hope you guys help me..;

Code:

class Post extends Controller
{
    function Post()
    {
        parent::Controller();
    }


    function product()
    {
        if(isset($_FILES['pdf']) AND strlen($_FILES['pdf']['name'])>0)
        {
            $pdf_path    = array('./pixs/products/pdf');
            $pdf_files    = array('pdf'); // FILE INPUT NAME
            $PDFUpload    = $this -> UploadPDF($pdf_path, $pdf_files)
        }
        
    }    

    function UploadPDF($upload_path=array(),$files=array())
    {
        $config['allowed_types']= 'pdf';
        $pdfPath[]        = array();
        $data[]            = array();
                        
        $count_files         = count($files);
                        
        for($i = 0; $i < $count_files; $i++)
        {
            $config['upload_path'] = $upload_path[$i];
            $this -> load -> library('upload',$config);

            if(isset($files[$i]) || is_array($files[$i]))
            {
                if(!$this -> upload -> do_upload($files[$i]))
                {
                    echo $this->upload->display_errors('<p>', '</p>');
                }
                else
                {
                        $data[$i]     = array('upload_data' => $this->upload->data());
                    $pdfPath[$i]     = $data[$i]['upload_data']['full_path'];
                                            
                    return $pdfPath;
                }
            }
        }
        return FALSE;
    }
}