You can download the latest man-pages tarball here; older tarballs can be found here. The tarballs for the man-pages-1.* releases can be found here.
Using Git, you can obtain the very latest version of man-pages from the canonical git repository on kernel.org using the following command:
$ git clone https://git.kernel.org/pub/scm/docs/man-pages/man-pages
You can see what's recently been pushed to kernel.org via CGit: https://git.kernel.org/?p=docs/man-pages/man-pages.git.
Since the 2011 kernel.org outage, there is also a secondary Git repository at https://github.com/mkerrisk/man-pages. (Please don't send GitHub pull requests or report issues via the GitHub issue tracker. See here instead.)
the IEEE and The Open Group have kindly granted us permission to distribute extracts from the latest version of the POSIX.1 standard:
IEEE Std 1003.1-2017, Standard for Information Technology--Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group.
The POSIX manual pages can be downloaded here.
The Git repository for the POSIX manual pages is here.