Hola albertcito,
Solo es cuestión que cambies la extensión en filename=$filename para que sea htm:
Código PHP:
<?php
$filename = "pagina.php";
$size = filesize($filename);
$temp = explode( "." , $filename );
$html = $temp[0] . ".html";
header("Content-Transfer-Encoding: binary");
header("Content-type: application/force-download");
header("Content-Disposition: attachment; filename=$html");
header("Content-Length: $size");
readfile("$filename");
?>
Saludos.