Ubuntu appears very small at 1080p, text almost undreadable

Using Ubuntu Tweak you can adjust the Text Scaling Factor. Increasing the value will increase text size system wide.

The setting can be found in Ubuntu Tweak<Tweaks<Fonts

If you don't have Ubuntu Tweak installed it can be installed by downloading the .deb file here, or you can run these commands in the Terminal:

sudo add-apt-repository ppa:tualatrix/next

sudo apt-get update

sudo apt-get install ubuntu-tweak

Try this: Open "System Settings" then from "System" section select "Universal Access". On the first tab marked "Seeing" there is a drop-down field marked "Text size". Adjust the text size to Large or Larger.

I had the same problem with an Asus Zenbook UX303UB. Couldn't change it neither with

gsettings set org.gnome.desktop.interface scaling-factor 1

nor with xrandr --dpi 138/eDP1, but:

For GNOME Shell (17.10+):Settings > Devices > Displays, 200%

For Unity (17.04-): All settings > Display, on Scale for menu and title bars set to 2.

However, some programs still look small.