In case you want to change away from the default theme of PDF Viewer or if you want to comfortably read books at night, PDF Viewer comes with a second Dark Theme and a Night Mode for documents.
The hand-crafted Dark Theme is perfectly suited for darker reading environments, for example at night, or if you simply prefer dark user interfaces. You can access it quickly from the Settings Screen of the app.
Launch the app or navigate to the main screen.
Navigate to Settings within the primary app bar.
Tap on Theme and select Dark Theme.
The Night Mode inverts all colors of your PDF document. Since most PDF documents have a white background, this feature can be used to "tune down" the brightness of your PDF while keeping the contrast at a maximum. Night Mode can be enabled directly in the viewer while reading a PDF:
Tap the View Options icon in the primary toolbar (it looks like a cogwheel).
Under Design select Night.