Yes, I do have a separate licence for Acrobat 9 but when I tried to install AcroPro90_efg.exe, I was told that my licence was "invalid". I didn't mention it in my 1st post: I was sure that Acrobat, which is part of CS5, was simply missing from the installer and did not require a separate install. That's the part I didn't remember from the original installation (2010). So this solves the "missing from the installer" part. Thanks!
To sum it up, I have a valid licence for a CS5 installer that includes (only) Illustrator, InDesign and Photoshop but a supposedly invalid licence for the Acrobat installer, which IS part of CS5. Great!
If your separate serial number for Acrobat is gives you any issues, then your best bet is contact Adobe directly and ask them to look into your licensing and/or reset your activation count.
(via live online chat is usually the fastest/easiest)
They are the only ones who really have power or control in this regard.