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.
To trim a picture, select it and tap Crop picture. Drag the 4 corner dots, then tap Crop (or Reset to start again).
When finished, tap ⬇ Download at the top to save the edited PDF.