Any PDF that can be opened by Preview can also be ?written? on, although I normally use PDFPen. Although it is an indication that the form creators have considered the users when they automatically insert editable fields.
In Preview, click the "Show Markup Tools" icon that is just to the left of the Search field. You will get a new 'toolbar'. There three sections. In the middle section are tools for adding Shapes, Text boxes, Signing and Notes. Clicking the Text Box will insert one in the middle of the top page. With the word "Text" in it. You can then enter text, of course, but also move the box anywhere on that particular page (over a empty line, for example). You also have the standard text formatting tools available. Don't forget to save/print the PDF, as needed.