With an update from 11.6 to 11.7, the PDF24 tray icon is no longer appearing. The app does appear to be running in the background. Reverting to 11.6 fixed the problem.
OS is Windows 11.6. Installed using the .msi installer
If the tray icon is still missing after that, please check the following:
In the PDF24 settings, the checkbox in the section "Tray Icon" must be enabled. Administrators can also hide the icon with the registry value NoTrayIcon, which is worth checking with MSI deployments: https://creator.pdf24.org/manual/11/#41-common-registry-settings