iAnnotate has a setting called "smart invert" that allows you to view your document in a dark mode.
To enable Smart Invert follow these steps:
1. Tap on the orange tab of the document you would like to view in dark mode so that a menu drops down
2. Select "Enable Smart Invert"
This is a per-document setting and not an app-wide setting, so you will need to do this for each document you wish to view inverted.
To disable Smart Invert follow these steps:
1. Tap on the orange tab of the document you would like to view normally so that a menu drops down
2. Select "Disable Smart Invert"