Thank you for your quick answer.
Unfortunately, holding CTRL with a two finger up is unpractical, and pdf's become extremely blurry by using this command.
Is there another option? I know it is possible, because the excellent new app 'Found' (www.foundapp.com) has this feature: it adapts the preview automatically to the size of your screen. But I had to uninstall Found because it only searches in titles, not in documents itself.
FYI... Spotlight is a file finder, not a file viewer. Spotlight was designed to allow you to quickly LOCATE anything on your Mac, so that you can OPEN IT afterwards.
What you need is a Spotlight Alternative, something like EASYFIND, click on the following link for more info on the app.