Thanks for the quick reply (and the plugin)!
But my WordPress installation seems to be working fine (I haven’t yet bothered to change much beyond the standard theme..):
https://www.10milesfromnowhere.nl/blog/
Could the problem be that the downloaded files are not in (child of) the /blog/ directory? So they’re from index.html and not blog/index.php ?
As soon as I change the .htaccess and click on a PDF link on the ‘main’ site, it says “doesn’t exist on server, 404 ” , when I delete the rule from htaccess, the PDF links work again.