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.

Did you know you can add a text box filled with color or a unique shape to your Google Doc? Here's where you'll find that feature in Google Drive.

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.