You can't change a link from Public to Personalized after it's been created.
If you want notifications, you should send your links as Personalized. Note that if you select that option and send to multiple recipients, each will get a unique link, so when the file is viewed or downloaded, you will know whose link was used. Note that there is no sign-in required, so whoever has access to that link will be able to download the file(s). In other words, if a recipient of a Personalized link forwards that link to another, any downloads/views will appear to be done by the person to whom you sent the link.
Hope this makes sense!