As explained by others, you have to un-gzip it first. They use a Content-Encoding: x-gzip header which is not really RFC conform.

But with the above mentioned approach, you can also download a complete copy of the ppm archive. With a little regex magic on the xml file(s) to correct the paths, you can go ahead and tell the (graphical) ppm interface "Hey, you know what, there's this new server called 'C:\myserver', check it out!".