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

Go to Settings, than Advance Options, and then scroll down to EBook UI and change the font size. Default is set at 12.5. I find it very comfortable to read after changing it to 20. And don’t forget to save the changes, after which you have to reopen the EPUB file again to the the change.