I'm using ArcMap 10.8 and I chose paper size ISO A0 Landscape in my layout and paper size A3 in printing setup, now the basemap looks like what's shown in the screenshot and when I try to change the settings back it still does look the same, also when I try to print to PDF it prints a corrupt file.
I have already reset ArcMap and PC but that didn't fix the problem.
Don't use any ArcMap printing functions, not even print to PDF. Just export to PDF and then print it.
Also your A0 map would just get resampled to A3 size and look awful with the imagery. You have to have the layout sized according to how it will be printed - if you don't want it resampled.