SUMMARY
A PDF is not correctly printed with 100% (it's slightly larger in Okular) and when changing the zoom settings in the printer driver no change is made to the print, so the scale cannot be corrected.
This occurs with a PDF document containing a 1:25000 map with a 4cm grid in Okular but not in other PDF readers


STEPS TO REPRODUCE
1. create a PDF with a map (or other picture) with grid of a specific size
2. Print at 100% scale and check the grid size is correct.
3. Print document at any other zoom setting and check that the scale has been changed.

OBSERVED RESULT
At 100% scale, the scale is not correct, and when changing the scale in the printer driver the scale stays the same as with 100% scale.

EXPECTED RESULT
In other PDF readers the scale is correctly printed and if the scale is changed in the printer driver, then the printed scale has also changed accordingly.

SOFTWARE/OS VERSIONS
Windows: 10
macOS: 
(available in the Info Center app, or by running `kinfo` in a terminal window)
Linux/KDE Plasma: 
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION
I compared the incorrect results of Okular with the correctly printed scale in Nuance PDF reader. The printer was a Brother MFC589ocn.