How to keep Word form fields editable after saving as PDF

I'm creating a series of Word templates to be used as fillable offer letters for HR staff to fill out, but once they are filled in, they need to be sent to the candidate as PDFs. However, they need to remain signable with digital signature for the candidate, and we cannot ask our HR staff to add signatures in Acrobat. How can I set up a Word template so that once the HR staff has filled in all the form fields; e.g., candidate name, salary, position, etc., the editable signature field will export to PDF and the candidate can e-sign the PDF? (And it would be great if they could put the signing date in a date field that also exports.)