PDF different size?

[suomi]

Hi all,
I can not get this problem right, I am doing something wrong but don't know what.
Setting the size on 325.5 mm x 210.0  extra space on 3mm all around.
When I export this to PDF4 press incl extra space (3mm) it gives me a different size in PDF.
The PDF comes out in 345.6 x 230, it is a mystery for me to solve this. Were does the 20 mm comes from?
Do someone has a solution or has the same experience, and above all, how to get the right size?
I would be very grateful, thanks.
Greets Willem Mulder (Netherlands)
Using macOS High Sierra 10.13.6 and Scribus 1.4.8

[Nermander]

It is important to understand how the PDF box model works.

https://www.prepressure.com/pdf/basics/page-boxes

When you set the page size, this is the TrimBox.

When you add 3 mm bleed (if that is what you did), this becomes the BleedBox.

These boxes will be in the PDF you export and a RIP or impositioning software should honor these.

But when Scribus exports a PDF it may set the MediaBox larger than the BleedBox to be able to fit for example crop and fold marks.

For any software capable of honoring the different boxes, this should not matter, because thanks to the TrimBox an BleedBox the contents should be correctly printed. Any content outside the BleedBox may be cropped away.

[suomi]

Thanks Nermander, great help, it is more clear to me now.