There is no document preview content in Document Preview panel. It shows ``` Don't know how to show mimetype 'application/pdf'. Please install Okular to make use of its PDF viewing component ``` But I've installed Okular and configuration is set.
KBibTeX 0.6 is still KDE4-based, so you need an Okular based on KDE4 as well. As far as I can tell, Okular in ArchLinux is KF5-based. To make the document preview work, you would either need to install Okular for KDE4 (which may no longer be available as distribution-provided package), or use KBibTeX from Git (master branch), which is already KF5-based and will make use of Okular for KF5. I have a prototype for a PKGBUILD file for ArchLinux, which I may append to this bug report for your to test.
Created attachment 105800 [details] ArchLinux package file for KBibTeX from git (master)