I try to display a pdf document with code like this:

<?php
    session_start();
    if (empty($_SESSION['nip']) AND empty($_SESSION['password'])) {
        ?>
        <script type='text/javascript'>
            alert('You must login');
            document.location='../../../login.php';
        </script>
        <?php
    }else{
        include_once '../../../config/core.php';
        include_once '../../../config/functioncollection.php';
        $kode_materi=antiInjections($_GET['id']);
        $querambildata=mysql_fetch_array(mysql_query("SELECT * FROM materi_tb WHERE kode_materi='$kode_materi'"));

        $file = "../../../filemateri/".$querambildata['url_materi'];
        $filename = $querambildata['url_materi'];
        header('Content-type: application/pdf');
        header('Content-Disposition: inline; filename="' . $filename . '"');
        header('Content-Transfer-Encoding: binary');
        header('Accept-Ranges: bytes');
        @readfile($file);
    }
?>

but when I opened, even download the file. how not to automatically download but just open the pdf file. thank you in advance

Your file name ends with '.pdf' correct? Everything looks okay to me after that. You might want to add this as well.

header('Content-Length: ' . filesize($file));

See if that makes any difference.

Make sure you're testing in more than one browser/machine, it might just be your machine that is doing it. Ultimately, the behavior is decided by the browser/software settings on the pc I believe.