Displaying PDF

I've been getting errors in my php_error_log file. The error occur in pairs (same timestamp). But my application runs fine.

[15-Jun-2018 11:33:42 America/New_York] PHP Fatal error:  Uncaught ErrorException: Cannot modify header information - headers already sent in /opt/lampp/htdocs/system/Debug/Exceptions.php:139

Stack trace:
#0 [internal function]: CodeIgniter\Debug\Exceptions->errorHandler(2, 'Cannot modify h...', '/opt/lampp/htdo...', 139, Array)
#1 /opt/lampp/htdocs/system/Debug/Exceptions.php(139): header('HTTP/1.1 500 In...', true, 500)
#2 /opt/lampp/htdocs/system/Debug/Exceptions.php(200): CodeIgniter\Debug\Exceptions->exceptionHandler(Object(ErrorException))
#3 [internal function]: CodeIgniter\Debug\Exceptions->shutdownHandler()
#4 {main}
  thrown in /opt/lampp/htdocs/system/Debug/Exceptions.php on line 139

The relevant code called from a controller is:

PHP Code:

...
header('Cache-Control: private');
header('Content-type: application/pdf');
ob_clean();
flush();
readfile($file);
... 


Any ideas?

Simpler is always better