PPD files from printer manufacturers that are under a free license.

For example, the original MIT license, shown for example under
http://www.opensource.org/licenses/mit-license.php, is okay but not an
often used modified MIT license, which does not allow redistribution if
the file was altered in any way from its original form.

If you have a PostScript printer and there is no PPD file included in
this package, ask your printer manufacturer for a PPD file or visit
http://www.linuxprinting.org/ppd-doc.html.