I had been able to add a signature to a PDF a week ago. But then someone sent an encrypted document that I couldn't add a signature to. Since then I have not been able to add a signature to any PDF. I've tried to change the saved signature but each time I get to that screen Adobe crashes.
I'm on a Mac with OS X 10.11.1
By "Adobe crashes" do you mean Adobe buildings ? Nah, you mean Acrobat or Reader, one of Adobe's many products. If you get a crash I suggest to submit a bug report to Adobe Feature Request/Bug Report Form.