There is no “password” on the Acrobat Pro application.
Perhaps you are referring to the serial number necessary to install the software? If you have lost your serial number but have registered your program with Adobe, log onto the Adobe website, click on Manage Account, and from there you should be able to retrieve the serial number for that and other products you licensed directly.
If you are talking about a PDF file protected with a password, that is something that is document-specific and for which you will need to get the existing password from whoever sent you the PDF file.
- Dov