How to find the serial number will depend on the way Acrobat Pro 9 was acquired. If the product was purchased from the Adobe website, log in to the account that was made when the product was purchased and then look in the My Products part of the My Adobe page to view all of the products that have been purchased and their serial numbers. If unable to remember the log in information, check the email address used to purchase the product because the serial number will be in the confirmation email Adobe sent.
If the product was purchased from a store, then the product's serial number should be located either on the box, printed on the CD case or shown on a piece of paper in the box. If the product purchased was a teacher or student version, then there are two ways to find the information. If the teacher or student version was purchased directly from the website, then there is not a serial number, but if it was bought in the store, the serial number will either be on the box or inside the box.