That would be:
<php if (!is_page('slug-of-your-page')) echo '<meta name="viewport" content="width=device-width, initial-scale=1" />' ?>
Beware that if you change the theme files, your changes will be lost if the theme gets updated.
That would be:
<php if (!is_page('slug-of-your-page')) echo '<meta name="viewport" content="width=device-width, initial-scale=1" />' ?>
Beware that if you change the theme files, your changes will be lost if the theme gets updated.