I'm very new and have a question regarding copyrights.
Would it be appropriate to download a model and print it with a 3d printer as a part of a project presentation in the university?
It is not correct that there are no copyrights on this site.
Without the express authorization of the author, no model can be reproduced.
The models uploaded to Grabcad can be downloaded for free, but this does not imply that (1) they can be used commercially, or (2) they are included as part of another model for commercial purposes, or (3) they are reproduced in 3D printing for their marketing, or (4) produce commercial renderings from uploaded CAD 3D models.