For a drawing that uses splines, e.g. by using the standard font, the exported PDF files are huge. I found a way to work around that in Linux. If the files are printed to a (PDF) file in Linux, the files are much smaller. For the two of my drawings that I checked, which both contain a moderate amount of text, the difference is 573% and 657%, respectively.
The reason for the difference comes from the fact that the printed, small, PDF:s contain splines, but the PDF:s exported via the PDF export function in QCad only contain straight lines, so each spline in the drawing is represented by a large number of straight lines.
Another drawback of the large PDF:s, apart from size alone, is that they take much longer to display and print.
I'm using QCad Professional version 184.108.40.206 on both Linux and Windows.