There's no direct relation between the MuPDF app you can download and KOReader. Basically they're both applications that use the MuPDF library to display PDFs (and other supported file formats).
All you need to do is long-press on a file.
Edit: I just realized that "MuPDF is still experimental" could be misconstrued although I think it should be clear in context. It refers to using MuPDF to open EPUB (and FB2) only. The MuPDF library has always been responsible for the PDF rendering part of kindle
pdfviewer.