ErrorDocument 404 http://www.votresite.com/spip.php?page=page404

<IfModule mod_rewrite.c>
        RewriteEngine On
        RewriteRule "\.(pdf)$" http://www.votresite.com/spip.php?page=page404
</IfModule>

<IfModule !mod_rewrite.c>
        <IfModule mod_access.c>
                <Files ~ "\.(pdf)$">
                        Deny from all
                        ErrorDocument 403 http://www.votresite.com/spip.php?page=page404
                </Files>
        </IfModule>
</IfModule>