When you export the PDF it will automatically be set to the 841 x 1189 paper size. You can tell the print shop it’s A0 (A-zero) paper or to print without any scaling.
In your first screenshot the viewport shows that the Camera properties have been modified from what is set in the scene. Be vary careful with this as it can create a mess for you if the camera gets reset. It’s better and safer to avoid overriding the scene’s camera properties.