as noted, reader is still available (see the link above).
the current version available for HP-UX is 7.09 (which isn't the latest). It lists the following requirements:
HP-UX
PA-8800 processor
HP-UX 11, 11i v1, 11i v2; HP-UX 11 users: GTK+ libraries 2.6.8 (required)
CDE Desktop Environment
512MB of RAM (1,024MB recommended)
Up to 330MB of available hard-disk space
Firefox 1.0 or later, Mozilla 1.73 or later
If you're only using it to filter the docs to a printable format (as shown in a previous post), then you may not need some of this (like the browser, cde and gtk), but I can't confirm that.
It will probably run on IA boxes under Aries, but I can't confirm that either.
To bad Adobe has seen fit to ignore HP-UX (at least it's later releases).