Very nice. It could only be made better by supporting more formats. Like mobi, fb2 and DJVU. I miss Sumatra when using a Mac as it could open all of these beautifully on Windows. Perhaps Sioyek could make a great replacement if coupled with a library like Pandoc.
Since this viewer uses MuPDF as the document engine, it should already support EPUB, MOBI, FB2, and XPS document formats.
If it doesn't, it should be a relatively simple task to upgrade the mupdf library to the latest version and recompile Sioyek to get all the file formats supported by mupdf.
