Plex86 software and related files are distributed under the LGPL (GNU Lesser General Public License). There should be a file 'COPYING' at the top of plex86 source tree containing this license. If not, you can obtain a copy from The Free Software Foundation, Inc . Everything in the distribution except the documentation tree is covered by the LGPL.
Plex86 documentation is covered by the GNU FDL (Free Documentation License). This encompasses everything in the 'docs' directory. There should be a file 'COPYING' at the top of the 'docs' directory, containing the GNU FDL. If not, you can obtain a copy from the link above.