Is it possible to make a PDF available in TUWEL, but prevent students from downloading and further distributing this PDF? They should only be able to look at it.

No, that's not possible.

As soon as you provide your students with content, no matter in what form, they will be able to distribute it. Even if there would be a possibility to e.g. include a PDF read-only, someone could still take a screenshot of it and then pass on the images.