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.

Devon Delfino/Insider 4. Click into the text box and add the desired text, then drag the box to the desired placement. Enter your text and move the box to the correct location. Devon Delfino ...

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.