Starting with PDFNet 6.3 there is a PDFViewCtrl.setColorPostProcessMode that allows you to have a night mode.
http://www.pdftron.com/pdfnet/mobile/docs/Android/pdfnet/javadoc/reference/com/pdftron/pdf/PDFViewCtrl.html#setColorPostProcessMode(int)
http://www.pdftron.com/pdfnet/mobile/docs/Android/pdfnet/javadoc/reference/com/pdftron/pdf/PDFViewCtrl.html#setColorPostProcessColors(int, int)
http://www.pdftron.com/pdfnet/mobile/docs/Android/pdfnet/javadoc/reference/com/pdftron/pdf/PDFViewCtrl.html#setColorPostProcessMapFile(com.pdftron.filters.Filter)