The librarian is simply a file store. From it's documentation in its source code:
The librarian is a file storage service for launchpad. Conceptually
similar to other file storage API's like S3, it is used to store
binary or large content - bug attachments, package builds, images and
Decoupling file storage from the rest of Launchpad presumably makes it easier to scale out. As far as I know, the urls are permanent, but it is not designed to be browsed by humans.
While there was talk of using the librarian for PPAs, I don't know if that has happened. You can see that the actual archives that contain the PPA packages can be found at:
ppa:andrewsomething/typecatcher is at: