Thanks a lot for your detailed answer, Michael.
It works much better with PNG, indeed. The image I get after inserting into Word is clearly readable when enlarging it to the whole page and zooming to 120%.
However, I still get truncated labels. Only one or two words here and there, but still, not very convenient. Please see the following uploads - the text is in French but you'll be able to find out the "..." in 7 different nodes: