Tap on the "T" on the toolbar above the viewer. Then click on page where you want to add a text box. You can start typing when you see the cursor shows up.
To change font, text color and size, tap on the error under the "T".
To resize the text box, tap on the box and adjust by its corners.
To delete a text box, right click (or long press on touch screen) and select "Clear Markup". The text box will be deleted.
Attention: You cannot undo delete.