Update: There is now a Knowledge Base article about updating older versions of Acrobat/Reader: http://helpx.adobe.com/acrobat/kb/update-patch-acrobat-reader-7.html
You may remember that Adobe “end of life’ed” Adobe Acrobat 9 more than a year ago. This does not mean that Acrobat 9 does not work anymore, it means that you will not get any security updates for new security threats.
When you install a new version of Acrobat, you always end up with a x.0.0 version (e.g. 9.0.0 for Acrobat 9), this means that you need to first upgrade your installation to the latest available version.
Up until recently, it was no problem to download all the old Acrobat 9 updates, but Adobe – because Acrobat 9 and older are no longer supported – removed those update links from its update web pages for Mac and Windows. The only update versions available are for Acrobat X and XI.
How does one update an Acrobat 9 version that needed to be re-installed? According to what I’ve heard from Adobe, it should still be possible to use the “Check for Updates” function in Acrobat’s Help menu, but the last time I’ve had to re-install Acrobat 9, that was not the case.
If you need to access updates for older (non-supported) versions of Adobe Acrobat, these are still available, but accessing them is a bit more complex now: Adobe has a FTP server that holds all old updates. The problem is that it does not have the release notes that go with these updates, this means that it may not be obvious which version of Acrobat is required for which update. We also don’t know what exactly was fixed in each update, but as long as we start out with a brand new Acrobat 9.0.0 system (e.g. installed from your original installation disk, or via the download page http://helpx.adobe.com/acrobat/kb/acrobat-8-9-product-downloads.html), and then install every available update in the correct order (e.g. 9.0.1 first, followed by 9.0.2 and so on), we will end up with a working system.
You can access these updates via these two links (one for the Mac, and one for Windows):
Your browser should be able to deal with FTP links, just like it does with HTML links. However, navigating the directory hierarchy is more like navigating a folder structure on your computer than selecting links on a web page.
You can also find updates for older versions of Acrobat on this FTP server.