Add Metabox to all custom post types

The 4th parameter for add_meta_box() is the post_type. So if you’re calling it with post, it’ll only show on WordPress’s native posts.

Try this:

add_meta_box(
    'my-meta-box-id2',
    'Enter your PDF location for your post category below:',
    'cd2_meta_box_cb',
    'your_custom_post_type_name',
    'normal',
    'high'
);

If you have multiple post types you’d like to attach this to, this should work:

$post_types = array( 'post_type_1', 'post_type_2', ); // and so forth
foreach( $post_types as $post_type) {
    add_meta_box(
        'my-meta-box-id2',
        'Enter your PDF location for your post category below:',
        'cd2_meta_box_cb',
        $post_type,
        'normal',
        'high'
    );
}

(Whoops — fixed the code to actually use the $post_type variable.)

Reference

Codex page for add_meta_box()