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.
Tap Shapes, then choose Box, Circle, Line, Arrow or Text box and drag on the page. Only one shape is drawn at a time: tap Done, then choose the next one.
To change colour tap a colour circle in the panel. Fill colours the inside, and Thin / Medium / Thick changes the line width.
When finished, tap ⬇ Download at the top to save the edited PDF.