If you've purchased ACDSee 3.1, your serial number can typically be found in the confirmation email or in the product packaging.
Because ACDSee 3.1 is over 20 years old, it may struggle with modern file types and high-resolution displays. Current options include: Acdsee 3.1 Serial Number
If you are looking for the speed and simplicity that ACDSee 3.1 provided, there are better, safer, and more modern ways to achieve that: If you've purchased ACDSee 3