I have managed to clean up the display of membership lists (civireport) to the satisfaction of our users by linking a CSS file to the report header, but I do not find how to adjust the font and layout of generated PDF (is that even possible?)
The PDF generator uses the HTML to create the PDF. So changing CSS will have effect. But.. to have a different font in the PDF, the font should be available on the server. If the font is not available on the server (and the pdf generator) then a fall-back font is used.
Here's some more info about DOMPDF (the tool CiviCRM is using for creating PDF's):
Did you look at print.css? It is in the civicrm/css folder.
You need to use inline css. I think it has something to do with Civi using mail framework to generate its templates.