POSIX pricing and availability; or: Do you really need the PDF?
The Open Group and IEEE are weird about POSIX pricing. They’re protective of the PDF, making you pay hundreds of dollars for the PDF; but will happily post an HTML version for free both online, and (with free account creation) download as a a .zip.
They also offer a special license to the “Linux man-pages” project, allowing them to distribute the man page portions of POSIX (most of it is written as a series of man pages) for free; so on a GNU/Linux box, you probably have most of POSIX already downloaded in manual sections 0p, 1p, and 3p.
Anyway, the only thing you aren’t getting with the free HTML version is a line number next to every line of text. It’s generated from the same troff sources. So, in an article or in a discussion, I’m not cheating you out of specification details by citing the webpage.
If you’re concerned that you’re looking at the correct version of the webpage or man pages, the current version (as of February 2018) of POSIX is “POSIX-2008, 2016 edition.”