SUMMARY
- With Okular, I am not able to print a pdf (page size A4) in 100% on my paper printer. My printer is a Brother MFC9142CDN, CUPS wrapper, network printer. It always scales to the printable area.
- LibreOffice 6.1.2.1 is able to print in original size. Yet I had to install the manufacturer's proprietary printer driver (with CUPS wrapper) to be able to get an originally sized output.
- Other pdf viewers, e.g. evince, also fail.
- Printing to a pdf file in KDE always works fine.

STEPS TO REPRODUCE
1. Install a network printer.
2. Produce a document with a graphical object in LibreOffice, export as Pdf.
3. Print from LibreOffice (set original size output) and from PDF viewer, print to printer and to Pdf file, try modifying paper margins. Measure the graphical object's size on paper with a ruler, in the printed Pdf with a Pdf viewer that features a ruler (e.g. LibreOffice Draw).

OBSERVED RESULT
I could only succeed with printing from LibreOffice using a proprietary printer driver for my network printer.

EXPECTED RESULT
- Scaling option in print dialog
- paper print in original size

SOFTWARE/OS VERSIONS
Windows: 
MacOS: 
Linux/KDE Plasma: KUbuntu 18.10 / KDE 5.13.5
(available in About System)
KDE Plasma Version: 5.13.5
KDE Frameworks Version: 5.50.0
Qt Version: 5.11.1

ADDITIONAL INFORMATION