Earlier this year several sources reported that Adobe was making their old CS2 software available for a free download. Immediately after the announcement the download page got slammed and was unavailable.
After checking in on it now, it looks like it is still available here. On the page it states this:
Adobe has disabled the activation server for CS2 products, including Acrobat 7, because of a technical issue. These products were released more than seven years ago, do not run on many modern operating systems, and are no longer supported.
Adobe strongly advises against running unsupported and outdated software. The serial numbers below should only be used by customers who legitimately purchased CS2 or Acrobat 7 and need to maintain their current use of these products.
That seems to say that you are supposed to buy a real version, but you can download it regardless of if you did. Also, it warns that it might not work on your computer. I tend to agree with the statements above. It is probably easier and safer to just buy the newest versions, but I guess this “free” option is there as well.