Is there a way to increase size and resolution of pdf-previews in the preview-window? jpg previews are ok, but pdf-previews have very low resolution and size so they are not readable on my 4k monitor.
kubuntu 16.04 with GTX-1050TI and kdegraphics-thumbnailers

Thanks for any hint

For me PDF previews are crisp and clear. I also have 4K UHD screen and I use scaling ratio of 1.5

What I noticed that when you scale your display via "Scale Display" in "Display and Monitor" settings, then in some places you get pixelated graphics, for example in Gwenview. So there I keep scaling ratio of 1 (default) and I just change my font DPI to 144 (default is 96). This way Qt scales most of things pretty well.

So I would recommend you to try keep scaling ratio in "Scale Display" to 1 and change Fonts DPI to 144 or 192 or more, depending on how much you want to scale whole UI.