Thanks Paradox, PDF preview sounds great, what about images?
Ok, let me first try to explain the actual behavior in more detail:
I want to share a folder with some JPEGs and PDFs with a link. No registration needed. When the recipients open the link, they will see all the images and PDFs in list or thumbnail view. Great so far. But when they click on an image or PDF it immediately starts downloading the clicked file. Instead of opening and previewing it in the browser.
In my installation every registered user which follows the shared link can preview the files directly in the browser but non registered users can only download them, no preview possible. Is this a problem of my Nextcloud installation or is this a normal behavior?
Maybe you want to try this behavior on your own: Create a shared link from a folder with images and open it in incognito or private mode of your browser where your are a not a registered user.