How can I add relative path to a pdf file link in drupal menu
The thing with the menu system is that what you are typing is not a relative path, but an internal Drupal URL.

Since what you are after is not a Drupal path, but instead a static file server outside of Drupal by your webserver your options are limited.


You can write the full url like you tried yourself
You can create a module/theme where you can make this possible, either by adding the extra link or creating a drupal path that redirects to the static file.


Of these, I personally think that a an absolute path is the lessor of evils, unless you need to do this a lot of times.