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.

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 ...

Adding Text Notes to a PDF Select the Text Box tool from the Drawing Markups panel. Click, or click and drag, on your document to add a box with a colored border, and type your observations in it.

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.

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.