I have started a new drawing with standardiso template. Then I go to Sheet 1 create a new configuration from default selecting Microsoft Print to PDF, A3 Landscape. The preview shows dimensions of 297 x 420. When I measure the width on screen it shows as 16.54. So it appears to be converting mm to inches. Even the font which should be 2.5mm appears to be 2.5 inches.

Have I accidentally activated an automatic units conversion? 

What is going on here and how do I fix it?