united-states
TL;DR: This is a fine distinction that courts have not yet clearly made in well-established caselaw. To the extent that Adobe's copyright is valid, it is likely rather narrow and may be subject to significant limitations and exceptions, especially fair use.
Quoting from the specification:
The general idea of using a page description language is in the public domain.
Anyone is free to devise his or her own set of unique commands that constitute a
page description language. However, Adobe Systems Incorporated owns the
copyright for the list of operators and the written specification for Adobe’s PostScript language. Thus, these elements of the PostScript language may not be copied without Adobe’s permission.
Adobe claims copyright over two things:
- The "list of operators" used in the language.
- The "written specification" of the language (i.e. the text, layout, formatting, etc. of the specification as a whole).
In general, their claim to (2) is probably stronger than their claim to (1), but that does not mean their claim to (1) is necessarily invalid. The rest of this answer will focus on (1), because I do not believe that anyone seriously questions the validity of (2).
Two cases are relevant here:
- Baker v. Selden, in which Baker copied some functional elements of an accounting scheme originally described by Selden.
- Google v. Oracle, in which Google copied some portions of the Java API, which Oracle claimed copyright over. The opinion quoted below refers to those portions as "declaring code," because they "declare" how the API is structured.
(Disclaimer: I work for Google, but not on Android, and I had no involvement with any of issues in Google v. Oracle.)
In Baker v. Selden, the court held that, even though Baker's book presented substantially the same accounting system as Selden's book, Selden could not claim ownership over the system itself, because only a patent could confer such protection. As the Supreme Court explains:
The copyright of a work on mathematical science cannot give to the author an exclusive right to the methods of operation which he propounds, or to the diagrams which he employs to explain them, so as to prevent an engineer from using them whenever occasion requires. The very object of publishing a book on science or the useful arts is to communicate to the world the useful knowledge which it contains. But this object would be frustrated if the knowledge could not be used without incurring the guilt of piracy of the book. And where the art it teaches cannot be used without employing the methods and diagrams used to illustrate the book, or such as are similar to them, such methods and diagrams are to be considered as necessary incidents to the art, and given therewith to the public -- not given for the purpose of publication in other works explanatory of the art, but for the purpose of practical application.
Subsequently, Congress codified this limitation into law at 17 USC 102(b):
In no case does copyright protection for an original work of authorship extend to any idea, procedure, process, system, method of operation, concept, principle, or discovery, regardless of the form in which it is described, explained, illustrated, or embodied in such work.
In Google v. Oracle, the Court of Appeals for the Federal Circuit determined, first, that the API was subject to copyright protection (overturning a lower court decision) and then (on a subsequent appeal) that it was not subject to fair use as a matter of law (again overturning a lower court decision). The Supreme Court overturned the latter opinion, finding fair use, but the wording of its opinion also cast doubt on copyrightability (which the Supreme Court expressly refused to decide):
These features mean that, as part of a user interface, the declaring code differs to some degree from the mine run of computer programs. Like other computer programs, it is functional in nature. But unlike many other programs, its use is inherently bound together with uncopyrightable ideas (general task division and organization) and new creative expression (Android’s implementing code). Unlike many other programs, its value in significant part derives from the value that those who do not hold copyrights, namely, computer programmers, invest of their own time and effort to learn the API’s system. And unlike many other programs, its value lies in its efforts to encourage programmers to learn and to use that system so that they will use (and continue to use) Sun-related implementing programs that Google did not copy.
Although copyrights protect many different kinds of writing, Leval 1116, we have emphasized the need to “recogni[ze] that some works are closer to the core of [copyright] than others,” Campbell, 510 U. S., at 586. In our view, for the reasons just described, the declaring code is, if copyrightable at all, further than are most computer programs (such as the implementing code) from the core of copyright. That fact diminishes the fear, expressed by both the dissent and the Federal Circuit, that application of “fair use” here would seriously undermine the general copyright protection that Congress provided for computer programs. And it means that this factor, “the nature of the copyrighted work,” points in the direction of fair use.
Since the Supreme Court declined to give a definitive ruling on copyrightability, we cannot pinpoint the boundary between a weak copyright and no copyright at all. What we can say is that the list of keywords has many of the same features described above (i.e. it is a user interface, it is "inherently bound together with uncopyrightable ideas," much of its value derives from use by others, etc.), and that may weaken the copyright, or possibly defeat it outright.
