if you want to change a file in the Publisher Library (e.g. add some information to a given PDF), you need to delete the old file and upload a new file. This new file has a different ID and thus the new version has a different URL than the old version.
If you already used the URL, e.g. on the website or in emails, you need the change the html-Code or you need to inform people about the new link.
It would be great to have a “Change File”-Button in the Publisher Library like there is one for galley files .