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 Picture, choose the photo (logo, stamp or photo), then tap on the page where it should go.
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.