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.

Need to edit a PDF document but don’t want to pay for the more expensive Adobe editors? You can add text to and sign a PDF with just the free Reader program.

Microsoft Edge provides a feature to highlight text in PDF and save highlighted PDF to your PC. This post shows you how to to highlight and save PDF using Edge.

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.