how do i link to pdf file in drupal 7
That's not a problem, Drupal stores uploaded files under sites/default/files by default. There's no reason file paths for uploaded files would be similar to the URL of the node they're attached to.

www.example.com/sites/default/files/ADMISSION_FORM.pdf is the correct path if you haven't changed any of the default settings. I'm not sure why you've mentioned security, as that really doesn't come into it.