There are two kinds of “form” files. One is a PDF file with fillable form fields, which enables you to fill the form directly by clicking the form fields with Hand tool, the cursor will appear for you to type in. This is called an Interactive PDF Form.
The other kind of form is a plain PDF file with lines and texts, which can be filled using the Fill & Sign tools or Comment>Typewriter in Foxit Reader. This is called a Non-interactive PDF Form.