Description:
------------
With some PDF files (not all), the filesize function report a file size that is 
not representative of the size of the file produced by readfile.

The filesize gives the same filesize as the original file. But the readfile 
doesn't give the exact same content as the original file, making it slightly 
bigger.

This results in some part of the file being cut if you include the filesize in the 
header.

I can reproduce this bug on 2 servers,

Test script:
---------------
header('Content-Type: application/pdf');
header("Content-Length: ".filesize($file));
readfile($file);
exit();

// THIS CODE PRODUCE THE SAME PROBLEM 

ob_start();
readfile($file);
$output = ob_get_clean();
header('Content-Type: application/pdf');
header("Content-Length: ".strlen($output));
echo $output;
exit();

Expected result:
----------------
Expect the filesize to return the right filesize. Or the readfile to output the 
exactly same content as the original file. 

Actual result:
--------------
The filesize function report a file size that is not representative of the size of 
the file produced by readfile.