Open Edit PDF text and choose your PDF. The editor opens with big coloured buttons at the top: Edit text, Add text, Picture, Sign, Highlight, Erase, Draw, Shapes, More.
Choose Shapes → Text box, drag a rectangle on the page and type inside it. The panel lets you change the border and text size.
Drag the item to move it. Drag the round corner dot to make it bigger or smaller.
When finished, tap ⬇ Download at the top to save the edited PDF.