Finally I can take screenshots easily on Linux! (Yes, it's always been possible, but I now realized how to do it)
I downloaded the software maim and bound it to a key combination.
When I press super + s now I can draw a rectangle and a screenshot of that will automatically be copied to my clipboard.
That's perfect as I can then paste it in Zettlr easily.
No more trying to save graphs and tables from pdfs, welcome to my life screenshot utility!