Attention: Terrain must not be moved!

1st step:
Place PDF correctly and activate Orthophoto window

Step 2:
Go to Top View --> Right click in orthophoto window --> Remove orthophoto "gray" --> "Calculate orthophoto" --> Place red area in new window and click "Calculate".

Step 3:
Go to 3D View --> Activate Terrain --> Set Texture Type to "Ortho ext." and select newly created orthophoto.

Result: PDF which is integrated in orthophoto on terrain