I've had decent luck with Xournal's "Annotate PDF" feature. It's not really interacting with the PDF form at all, so if the form is at all complicated it'd be

I've had decent luck with Xournal's "Annotate PDF" feature. It's not really interacting with the PDF form at all, so if the form is at all complicated it'd be a waste of your time. But, if you just need to "sign" something by dropping a transparent PNG on a line at the bottom and typing in a date, it's perfect.