Exporting to pdf

I have a problem which is affecting my ability to interact with trade printers.

When I export a file to Adobe Acrobat Pro DC pdf which is say 91x61mm, it is showing not as 91x61mm dimensions but 98x68mm.

If I do the same with a file size of 210x297mm it opens in Acrobat as 217x303mm.

As I type I this I have tried again and the results are now random sizes in Acrobat.

If I try Illustrator  - perfect!

Historically I need to use CorelDraw.

Is there anyone who can advise me on this issue please?

Thanks, Mike Healy