You need a special foot for free motion quilting--it is slightly shorter in height than a regular presser foot, which essentially provides space between the presser foot and the needle plate to manually move the fabric. Sometimes machines came with a standard darning foot accessory. Your Pfaff dealer probably has at least one style in stock. But there are other specialty feet you can probably use, ie Big Foot, clear free motion foot, C-shaped, oval, etc. You can also purchase a shank adapter (verify it is the correct shank for your machine) and use it with a kit of snap-on presser feet so you can have a variety of presser feet for different functions.

Pfaff Presser Feet

Pfaff Big Foot Free Motion Quilting Foot

Pfaff Free Motion Guide Presser Foot