How to create a link to a file in a module?
file_create_url() is right choice. Check the url it generates and try to open it directly in browser. If it fails do one of these:


Check if file is exists in right folder
If yes, check if it readable (filesystem permissions). Also check permissions for files/ directory.
If file exists and is readable, check your server setup. It should allow to view files from files/ directory. Probably you have wrong mod_rewrite config (apache) or wrong nginx config.