Sumatra PDF is a PDF, ePub, MOBI, CHM, XPS, DjVu, CBZ, CBR reader for Windows

Changing font size

There are no instructions for font size changes. Is it at all possible?
Thank you

Fonts etc. in PDF are by definition fixed by the author or their editor.

The one font used in SumatraPDF eBook interface (or fixed interface when used for eBooks and Chm) is by default Georgia and can be changed per,

https://www.sumatrapdfreader.org/docs/Customizing-eBook-UI.html