upload PDF file fail

Dear sir,
I can upload many kind of files but always fail when I upload the PDF file. The system reply me as shown as attached image file. Can you help me to debug it? My config.php as shoen below for your reference:

$config['DefaultResourceTypes'] = '';

$config['ResourceType'][] = Array(
'name' => 'Files', // Single quotes not allowed
'url' => $baseUrl . 'files',
'directory' => $baseDir . 'files',
'maxSize' => 0,
'allowedExtensions' => '7z,aiff,asf,avi,bmp,csv,doc,docx,fla,flv,gif,gz,gzip,jpeg,jpg,mid,mov,mp3,mp4,mpc,mpeg,mpg,ods,odt,pdf,png,ppt,pptx,pxd,qt,ram,rar,rm,rmi,rmvb,rtf,sdc,sitd,swf,sxc,sxw,tar,tgz,tif,tiff,txt,vsd,wav,wma,wmv,xls,xlsx,zip, vob, mp2',
'deniedExtensions' => '');

$config['ResourceType'][] = Array(
'name' => 'Images',
'url' => $baseUrl . 'images',
'directory' => $baseDir . 'images',
'maxSize' => "16M",
'allowedExtensions' => 'bmp,gif,jpeg,jpg,png,avi,iso,mp3',
'deniedExtensions' => '');

$config['ResourceType'][] = Array(
'name' => 'Flash',
'url' => $baseUrl . 'flash',
'directory' => $baseDir . 'flash',
'maxSize' => 0,
'allowedExtensions' => 'swf,flv',
'deniedExtensions' => '');