Note: in 3.4 we changed the eBook rendering engine and this customization is no longer possible.

Sumatra has 2 UIs:

As of version 3.1.2 eBook UI doesn't have zoom (changing font size), text selection etc.

There are 2 ways to customize eBook UI.

Use advanced settings

Use Settings -> Advanced Settings menu and find EbookUI section.

As described in settings you can change the following aspects of eBook UI:

Use PDF UI for eBooks

Use Settings -> Advanced Settings menu and find EbookUI section.

Set UseFixedPageUI = true