Forms on websites typically write to a database or flat file, not PDF. Users fill out the form and the signature field is no different, for example - here’s a Google form:
By typing your full name above, you are providing us with your digital signature, which is as legally binding as your physical signature. Please note that your signature must exactly match the first and last names that you entered at the top of this web form in order for your submission to be successful. https://www.google.com/webmasters/tools/legal-removal-request?hl=en&complaint_type=1&pid=0
Many companies will in turn email a PDF receipt to customers, for example Apple, from the submitted form.
Hi Lloyd,
Forms are a vast subject and big business for those in the field. There’s lots of commercial and open-source products to choose from, really depends on what you need. For example, Google Forms (which syncs with Google Docs) is more than adequate for many situations. In WordPress (along with the numerous free plugins like Contact Form 7), there’s popular commercial plugins such as Ninja Forms and Gravity Forms. The CMS Concrete5 has an excellent form builder too. And recently recommended on this board by several members is MachForm. Again, lots and LOTS to choose from.