Reading Mode makes the document more flexible and easier to read, especially on small devices. 

1.Tap the overflow button in the Viewer on the top right, then tap on View Mode. If you are using the Compact UI, then you will need to first switch to the View Toolbar. If you don't see the Action Bar, tap the center of your screen to bring it up.

2. Choose Reading in Viewing Mode section.

There are two methods to change HTML text size:

Method 1: Tap upwards/downwards arrow buttons in front of Text Size to increase/decrease the text size.

Method 2: Use the pinch in or pinch out finger gesture to increase/decrease the text size.

Note if the app is not able to extract any text from the document, for example when the document is a scanned image, the following message is shown.

This page doesn't contain reflowable text