Cheap (free) and easy solution: Formulate. It’s nearly perfect for filling in PDF character sheets, since you can save a filled-in sheet as a Formulate document and then use nifty built-in OS X features to print to PDF. For extra geek points, use handwriting fonts: there are a bunch here, amid the popups and blinking ads; Chank has you covered if you want to shell out for somewhat higher quality; or go grab the ECF handwriting fonts, which I like a lot.