This thread will get complicated enough without someone diverting us from tracking the versions of a specific piece of software. Please set up a new thread if you want to recommend Nitro, or to compare various PDF readers.
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They've been doing regular quarterly updates, apparently mostly bug and security fixes. So if you are sticking with version 9, it seems that you should keep up with its updates.
The new version 11 just got its first update, and version 10 just got another update. No new update for 9.5, but I think they're still supporting it. There haven't been updates for version 8 in over a year.
Updates (if any) are issued every quarter. The last updates were released on 9/10/13.
Here is the update download page for all supported versions:
http://www.adobe.com/support/downloa...atform=Windows
This page contains a link to the direct download page for the latest full installers, and then you can get updates from the previous link:
http://helpx.adobe.com/acrobat/kb/tr...n-windows.html
To update to 9.5.5, you need to install 9.5, then updates for 9.5.1, 9.5.2, 9.5.3, and 9.5.5. (The only one you can skip is 9.4.4.)
To update to 10.1.8, install 10.1.4 and then the 10.1.8 update.
You can get a full installer for 11.0.4.