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.
For a straight line choose Line and drag from the start point to the end point; the line stays straight.
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.