This post helps to add a text box to PDF using a free software and an online tool. You can adjust the size of text box, text style, etc.

To add your signature to another spot or a subsequent PDF, click on the Sign icon and choose your saved signature. After you’ve added text and signed the PDF, save the file.

You can type on a PDF using one of several different options, depending on the device you are using.

In other words, Adobe Reader won't let you add text. What you need is a program that's a little more accommodating, PDF-wise. Try Foxit Reader 4.3, which allows you to add text to any PDF.