URL Formula to open file attachment

Ok - I may be missing something easy here, but I need help.

I have a file attachement field that may or may not have a pdf attached.  Is it possible to have a URL Formula (so I can have a button) that will show up only IF there is an attachment and the URL would be a link to that attachment?

If it's not possible to have it show up only if there is an attachment - what would the formula be to link to that field either way?  - I know there is the link that is the file name, but I really just want it to show up as a button.

Thanks!