It has nothing to do with filepath. It changes the escaping behavior of strings.
In a string literal prefixed with @ the escape sequences starting with \ are disabled. This is convenient for filepaths since \ is the path separator and you don’t want it to start an escape sequence.
In a normal string you would have to escape \ into \\ so your example would look like this “pdf\\”. But since it’s prefixed with @ the only character that needs escaping is " (which is escaped as "") and the \ can simply appear.
This feature is convenient for strings literals containing \ such as filepaths or regexes.
For your simple example the gain isn’t that big, but image you have a full path "C:\\ABC\\CDE\\DEF" then @"C:\ABC\CDE\DEF" looks a lot nicer.
For regular expressions it’s almost a must. A regex typically contains several \ escaping other characters already and often becomes almost unreadable if you need to escape them.