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.

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.

Removing text from a PDF isn't as easy as drawing a black bar across it. Fortunately, redaction tools let you truly erase sensitive text in Preview, Adobe Acrobat DC Pro, and PDFPen.