Have you ever received an Adobe form that does not have interactive fields? You may have opted to just print the form and then pick up a pen to fill it in. Adobe Acrobat actually has a built-in tool to allow you to type directly on the form. It's simple to use: Just open the PDF document, then click on the TOOLS option from the toolbar and select TYPEWRITER.
Position the cursor on the page where you want to type. It is presumably in a box or on a line. The mouse will turn into an I-beam. Type your text.
You can even reposition the text clicking on it with the Typewriter selection. It is also possible to resize the box by dragging on the "handles". If the box is too narow to accommodate the text, Acrobat will wrap it into multiple lines.