If you are looking to simply manage your ebooks then I would recommend Calibre E-book manager it is simple, powerful cross platform and cross format tool for managing your ebook collection including tools for converting between many formats.
Regarding your specific needs:
- Conversion - Calibre provides conversion between many differing formats but there can be issues with a) DRM & b) PDFs - the content of a pdf varies depending on the producer and can be simple to convert (PDF contains text & simple layout), difficult (PDF Contains text in near random order and complex layout), or even require OCR (pdf from scanners contain images of pages),
- Managing - This is what Calibre was written for.
- Reading experience - there are readers for many formats provided within Calibre or installed platform software can be used how user friendly the reader is is subjective, varies between platforms and with installed software.
I hasten to add the I have managed to avoid Windows 8 other than removing it and replacing the Ubuntu in a couple of cases but you should have no problems running Calibre.
If you need to annotate and cross-reference within pdf files, rather than e-books, then Mendeley would possibly be a better fit as it allows you to highlight and cross reference within pdfs.
There is of course nothing to stop you using both.