It is short for "Piano Street Edition" and is sheet music we typeset ourselves.
The other file available is not really plain vanilla but a scanned public domain edition from the beginning of last century.
We will later on add information to the database of who the editor of those scanned editions are.
While we are talking about this anyway, you may also find files tagged "PS Urtext".
The difference between PS Edition and PS Urtext is that in the former there are some additions such as slurs, dynamics etc which is not written by the composer but may be of great help for unexperienced players. In the PS Urtext we try as far as possible to publish exactly what the composer wrote without any additions.
Take a look at the Bach Inventions we have
here for a clear example of the difference between Urtext and an edited score such as the Czerny edition.