This might be a very stupid question, but I don't manage to find the pgf manual
v2.00. On CTAN I only found
v2.10. (I'd like to find it because I want to follow Andrew's comment to this great answer.)
An alternative way is to use the TeX Live SVN repo: the root of the trees is at http://tug.org/svn/texlive/trunk/Master/. Then you append the location of the pgf manual:
texdoc -l pgf will show you that the file is there), and click the
pgfmanual.pdf link to see the list of revisions. Finally, use the "download" link for the revision you want: here the final url is http://tug.org/svn/texlive/trunk/Master/texmf-dist/doc/generic/pgf/pgfmanual.pdf?revision=6741.
This works fine for files whose location doesn't change too often.
You can get old versions of pgf at the pgf sourceforge site. The manual should be bundled with the rest of the files.