I'm trying to install Adobe Acrobat 9, however, I'm getting the message invalid serial number.

,,, what concerns answering: did you use a "special" word ... ?

... ahh there could be another thing:

If you purchased your product from these firms like Amazon, you "only" get a redemption-code with which you can request a serial number from Adobe. Please have a look at https://helpx.adobe.com/x-productkb/global/find-your-serial-number.html >>> (Start here: Prepaid card, product box, reseller)

The following part, so I just see at least, ceased to exist on my Adobe website, everything takes place in the link from above. I leave it as an info yet, it might still fit for you. For this purpose, please click your way through to your Adobe Store and find the button "Get Serial Number". Fill in the form and after a while you will get the real serial number.

Hello,

the easiest way: you should have a look at Adobe's database, to see what's stored/saved about your accounts. Here you will find general infos about your Adobe Account: https://accounts.adobe.com/>>> see My productsView all >>> These registered products are associated with your Adobe ID. >>> Name >>> Serial number.

And if of interest: to find your serial number have a look there http://helpx.adobe.com/x-productkb/global/find-serial-number.html  >>> I lost my serial number >> see comment. The last time Adobe suggests so too this

https://helpx.adobe.com/contact.html?step=ZNA_downloading-installing-setting-up_licensing- activation

Hans-Günter