为什么我在php中的返回文件在表单发送后不能工作？

<form action="process.php" method="post">
  <!-- 表单内容 -->
</form>

<?php
error_reporting(E_ALL);
ini_set('display_errors', 1);

// 处理表单数据
?>

$file = 'path/to/your/file.pdf';

if (file_exists($file)) {
    header('Content-Description: File Transfer');
    header('Content-Type: application/octet-stream');
    header('Content-Disposition: attachment; filename="'.basename($file).'"');
    header('Expires: 0');
    header('Cache-Control: must-revalidate');
    header('Pragma: public');
    header('Content-Length: ' . filesize($file));
    readfile($file);
    exit;
} else {
    echo "文件不存在";
}

ob_end_clean();

$file = 'path/to/your/file.pdf';
$filename = basename($file);

if (file_exists($file)) {
    header('Content-Description: File Transfer');
    header('Content-Type: application/octet-stream');
    header('Content-Disposition: attachment; filename="' . $filename . '"');
    header('Expires: 0');
    header('Cache-Control: must-revalidate');
    header('Pragma: public');
    header('Content-Length: ' . filesize($file));
    readfile($file);
    exit;
} else {
    echo "文件不存在";
}