Open <root_directory>/system/application/config/mimes.php file
Change value of ‘pdf’ element of $mimes array
$mimes = array( ‘hqx’ => ‘application/mac-binhex40’,
‘cpt’ => ‘application/mac-compactpro’,
‘csv’ => array(‘text/x-comma-separated-values’, ‘text/comma-separated-values’, ‘application/octet-stream’, ‘application/vnd.ms-excel’, ‘text/csv’, ‘application/csv’, ‘application/excel’, ‘application/vnd.msexcel’),
‘bin’ => ‘application/macbinary’,
‘dms’ => ‘application/octet-stream’,
‘lha’ => ‘application/octet-stream’,
‘lzh’ => ‘application/octet-stream’,
‘exe’ => ‘application/octet-stream’,
‘class’ => ‘application/octet-stream’,
‘psd’ => ‘application/x-photoshop’,
‘so’ => ‘application/octet-stream’,
‘sea’ => ‘application/octet-stream’,
‘dll’ => ‘application/octet-stream’,
‘oda’ => ‘application/oda’,
‘pdf’ => array(‘application/pdf’, ‘application/x-pdf’),
To
‘pdf’ => array(‘application/pdf’, ‘application/x-pdf’, ‘application/x-download’,’application/x-download’, ‘binary/octet-stream’, ‘application/unknown’, ‘application/force-download’),
nitinkelde
shahajip
starcoder
stardeveloper
Mumbai PHP Group 
For Multiple files uploading
load upload library once.
$this->load->library(‘upload’);
Set the different configurations for each file, initialize it and upload the file.
$config[‘upload_path’] = ;
$config[‘allowed_types’] = ;
$config[‘max_size’] = ; // Set 0 to get default value from php.ini
$this->upload->initialize($config);
$this->upload->do_upload();
&
$config[‘upload_path’] = ;
$config[‘allowed_types’] = ;
$config[‘max_size’] = ; // Set 0 to get default value from php.ini
$this->upload->initialize($config);
$this->upload->do_upload();
&
$config[‘upload_path’] = ;
$config[‘allowed_types’] = ;
$config[‘max_size’] = ; // Set 0 to get default value from php.ini
$this->upload->initialize($config);
$this->upload->do_upload();
and so on ….