What app are you using?
If you are reading in the iBooks app you can change the brightness by tapping on the AA icon. Most other app optimized for reading will have similar brightness controls, as well as themes like sepia or night mode.
Edit: When viewing a PDF in iBooks, it's a little sun icon. My mistake.
Failing that, you can always go to Settings > Display & Brightness to turn it down. If it Auto-Brightness is turned on the iPad will adjust to some extent, depending on how much light is detects in it's surroundings. That smaller dot next tot he front camera is a light sensor.
For quicker access, swipe up from the bottom edge of the screen for the Control Center. It has a brightness control slider. Control center may not be available in an open app. If it isn't you can turn it on in Settings > Control Center.