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.

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.