Josieb, why read PDFs in apps that aren't really intended for handling them? I assure you that if you insist on finding a
single, universal app to handle all those different formats, you're only punishing yourself. You will get a diminished reading experience from
all formats, instead of the best possible reading experience for every format.
For MOBI files, just convert them to EPUB using (for example) Calibre, which only takes a few seconds, and then read them in Marvin along with original EPUB files. I assure you that the Marvin reading experience is worth the few seconds needed to convert from MOBI to EPUB!
For PDFs, the world agrees that
GoodReader is the finest app to serve all your PDF needs, and it would be senseless for Kris to want to compete with it. I can also heartily recommend the
neu.Annotate app to you for reading, and especially marking up PDF files.