thanks, thomas. A little cumbersome but it works. As you say, I need to export the whole yEd diagram first to svg (disabling first the grid, otherwise it gets exported as well) and then open the svg file with, in my case, LibreDraw, to finally export it as a PDF.
I mean, this is ok if there are only a few images that will have to undergo this process. But for many, it's like doing the same thing, twice.