There are two kinds of PDF form files. One is a PDF file with fillable fields, which enables you to fill in the form directly by clicking the form fields without using other features. This is called an Interactive PDF Form. The other is a plain PDF file with lines and texts, which should be filled with the typewriter feature or by creating a fillable form in PDFelement. This is called a Non-interactive PDF form.
If a PDF form contains non-interactive form fields, you can fill in the form by clicking the "Form" > "Text Field" button to add text.