It looks like the Xpdf web page is inconsistent. I got this from the README:
License & Distribution
Xpdf is licensed under the GNU General Pulbic License (GPL), version 2
or 3. This means that you can distribute derivatives of Xpdf under
any of the following:
- GPL v2 only
- GPL v3 only
- GPL v2 or v3
The Xpdf source package includes the text of both GPL versions:
COPYING for GPL v2, COPYING3 for GPL v3.
Please note that Xpdf is NOT licensed under "any later version" of the
GPL, as I have no idea what those versions will look like.
If you are redistributing unmodified copies of Xpdf (or any of the
Xpdf tools) in binary form, you need to include all of the
documentation: README, man pages (or help files), COPYING, and
If you want to incorporate the Xpdf source code into another program
(or create a modified version of Xpdf), and you are distributing that
program, you have two options: release your program under the GPL (v2
and/or v3), or purchase a commercial Xpdf source license.
If you're interested in commercial licensing, please see the Glyph &
Cog web site:
Yeah, doesn't this require that all software that supports the format needs to be released as GPLv3 as well?
Who's bright idea was that?
The reference implementation is under GPLv3. Everyone is of course still free to create their own implementation and license it under whichever license they want.
So you released version 111 and some people downloaded it and purchased a support contract. Then you add a major new features in version 112 that due to their awesomeness have to break backward compatibility with version 111, and for that reason the aforementioned customer is not going to upgrade right away... but they did find a bug in version 111 that you are obligated to fix.
Myth number one, new features does not need breaking changes. Take a look at the Linux kernel. Probably the most modern kernel of all time, yet its developers take pride in that they don't break compatibility. New features does not have to come with the price of breaking changes.
It's not hard to admit errors that are [only] cosmetically wrong. -- J.K. Galbraith