My old boss in Tucson swore by a 6-inch drywall knife for trimming carpet pad | Carpet Installers - Freelance Forum - The Community for Independent Professionals
People get so intense about carpet pad tools. It's just padding, not a heart surgery. A sharp blade of any kind gets the job done if you know how to use it.
That drywall knife trick is solid for a straight line. Sometimes a sharp utility knife with a fresh blade works better on thicker pads though. Really just depends on what you're cutting.