White screen when output buffer php on some smartphone only - php

Question

I am writing a website using bootstrap 4, html 5 and php 7.
The website handle a folder with many pdf files inside.
In order to control the access to the pdf file, i use a php file that output the pdf wanted.
The folder of pdf is protected with a htaccess file.
Everything is working perfectly expect on some smartphones which display a white screen instead of the pdf file.
Same for the download option.
For debugging, I added logs at each steps,
In the normal situation, where i get the pdf, i got "-9-13-"
In the situation with the white screen, i got "-9-13-16", so something weird with the "exit"
Any ideas ?
if (file_exists($file))
{
//log_dbg('9-');
if(isset($_POST['download']))//download
{
//log_dbg('10-');
header('Content-type: application/pdf');
header('Content-Disposition: attachment; filename="' . $chapter_file_name . '"');
header('Content-Transfer-Encoding: binary');
header('Content-Length: ' . filesize($file));
readfile($file);
//log_dbg('11-');
exit;
//log_dbg('12-');
}
else //Stream
{
header('Content-type: application/pdf');
header('Content-Disposition: inline; filename="' . $chapter_file_name . '"');
header('Content-Transfer-Encoding: binary');
header('Content-Length: ' . filesize($file));
header('Accept-Ranges: bytes');
readfile($file);
//log_dbg('13-');
exit;
//log_dbg('14-');
}
}
else
{
//log_dbg('15-');
echo 'Fichier '.$file.' introuvable';
}
//log_dbg('16-');
Do you have any idea what can be the problem ?
You can try in real here :
http://laguildedesecrivains.fr/story.php -> click on "Chapitre 1"