High-DPI Displays under Linux
Although progress has been made to support HiDPI screens under Linux distributions, there are still some issues remaining. Jonathan Corbet discusses the state of HiDPI support on Linux in an LWN article. As he points out the GNOME and the KDE desktop environments pursue different approaches: GNOME implements pixel scaling, whereas KDE relies on display-independent units. GNOME's solution can be considered a short-term solution, while KDE's solution is sustainable. In order to activate pixel scaling in GNOME with a factor of two, the command gsettings set org.gnome.desktop.interface scaling-factor 2 can be used. KDE provides DPI settings for fonts in system settings under Application appearance -> Fonts and allows you to increase the icon size in system settings under Application Appearance -> Icons. A post on Ask Ubuntu explains how to determine the DPI for your screen and how to configure your X server accordingly. The Arch Linux Wiki summarizes configurations for several desktop environments, applications, and the Xft DPI settings. The tool font-config-info prints the font configuration settings.
In the browser world, recent Chromium versions lack HiDPI support under Linux, but the issue has been worked on. Firefox allows you to enable pixel scaling by setting layout.css.devPixelsPerPx to the desired factor in the about:config advanced configuration menu. Moreover, the issue can be mitigated using the Firefox add-ons AutoHiDPI or NoSquint. Recent versions of Opera provide HiDPI support.
In my case, besides tweaking the desktop environment settings, adjusting the lightdm.conf as explained in the Ask Ubuntu post and creating an .Xresources file containing the Xft DPI settings as described on the Arch Linux Wiki improved readability considerably.