在wordpress中获取上传的pdf文件的网址

i have added new custom field on post page whose type is "file" and his slug is "pdf_file".now i want to get url of uploaded pdf file form post. how is this possible for me to get the url of uploaded pdf file from post. i have tested following scripts but i am not getting url.

$my_meta = get_post_meta($post->ID, 'pdf_file', true);

AND

$my_meta = wp_get_attachment_url($post->ID);