1. Open Fill PDF form
Choose the form. Its fields become boxes you can type into.
2. Fill and flatten
When you're done, the answers are flattened into the page so they look the same in every viewer.
3. Sign if needed
Add a drawn or typed signature with Sign PDF. Check whether the receiver accepts it.
Questions
My form has no fields. What now?
Use Edit PDF to add text boxes where the answers go.
Related: Sign a PDF without printing it · Add a watermark to a PDF