So, you want to know about the license. Well, no need to worry, the pfe sources themselves are under LGPL - the Lesser GNU General Public License - which gives you lots of freedom just as long as you ensure that the library itself compiles from sources that are completly under LGPL - whatever changes you do and whatever other code you link the binary version of it to. An often used feature is the ability to add binary modules that extend the forth system, there are enough hooks in the pfe system to allow to change many internal characteristics of PFE, good ways to add closed source functionality through a backdoor. And that's okay as long as PFE can start up and work correctly without it.
For the documentation, I think the FDL (the Free Documentation License) is more appropriate. Oh well, please now, let's stop about all these license issues - if you have a cool application for the library, just ask and we can arrange for a special permission. No need to do like gforth, where 10% of the documentation file is just for explaining the "protection" from licenses - well, some people would even copyright garbage just to protect you from using it in the wrong places. (and don't do that at home).
The only thing that needs some explanation goes about the question who does actually have a copyright. The answer is simple - every file in the library does usually bear a copyright/author line near the top of the actual source file. This is the owner. The owner may even give another license to the file - there is just the implicit right to link it into an LGPL library that the PFE as whole represents. Some files don't have an explicit copyright statement, which will not make them public domain, just some appropriate license is imposed from the overall PFE package. And the whole of the PFE has three copyright holders - where acutally the history of the project defines these relationships, and not some legalese.
The first author of the PFE has been Dirk-Uwe Zoller, the inventor of the project. He did start with PFE around 1992 and he did work on PFE up to about 1995 with 0.9.14 being the last version. Well, Stephan Wenk of Siemens did some patches up to 0.9.22 however his version had not been seen widely, and in fact the newer PFE stems from duz' 0.9.14. In fact, I did contact Dirk a few times over the years but he does not have an actual interest about developing a forth system anymore, however he sometimes checks the comp.lang.forth newsgroup for references. Lately he opened an extra website, just have a look at www.dirk-zoller.de
The second author of PFE is Guido U. Draheim, however he did choose the PFE for a project request by Tektronix Berlin in 1998, and he is paid by Tektronix Berlin to work on PFE - and therefore the second copyright goes to Tektronix. In the year 2000, the PFE had been actually released to customers of Tektronix, and since the PFE is under LGPL, the customers have a right to get access to the PFE sources themselves. Guido did now take the chance to ask the Tektronix License Manager to be allowed to maintain an open-PFE version of the library - starting with the year 2000 release. Over time, quite some work has been done outside of the Tek Labs and the open-PFE project found a home at its own website at pfe.sourceforge.net From time to time, both the Tek-PFE and Open-PFE get merged together, just be aware that for both versions Guido U. Draheim is the main developer. In the Changelog file, the two roles are distinguished by using two mail-addresses - email@example.com and firstname.lastname@example.org - and these flag also the two different copyrights involved.
That's it. Any problems? Just contact me: "email@example.com"