Ok Kovidgoyal. I think I have solved it. Thank you for all your help and of course for Calibre. The setting that I found most suitable for converting epub to pdf for a4 printing seems to be:
Output profile - Default output profile
Override paper size
Set Paper size - a4
Margins
Top 20pt, bottom 40pt sides 25pt
Font
Serif family - Calibri
Sans family - Calibri
Default font size – 18
Monospace font size - 16
Page number
<p style="text-align:center; color:black">Page _PAGENUM_</p>