Hi, and welcome to the QCAD forum.
PDF export uses the print settings stored in the drawing.
See: Drawing Preferences .. Printing .. Page
Scale = 9.23 ; Offset (38.9112, 38.6241) ; A4
Your 30mm long line is printed as 276.9mm long on a A4 in portrait what is 297mm long.
Probably the drawing was already fitted to paper and centered before.
If we zoom in closely in print preview mode the line width is from 49.981 to 50.019 what is about 0.038 wide.
Multiplied by the paper scale that is 0.35074 what is a match with 0.35mm (ISO) give or take some measurements errors.
In that scale the page is just over 22.6 by 32 units what is again a reasonable match with A4.
So, no surprises there.
Using QCAD Command Line Tools is practically out of the question on my Win7.
Or
qcadcmd.com is not recognized or I get 'Access denied' and when I tweak all that then QCAD reports 'Cannot import file' ....
Please include example files as per forum rules, and in this case the PDF exported by
dwg2pdf
Regards,
CVH