I use both Okular and Calibre and as far as my usage goes, both of them fall into different niches. I find Okular excellent for reading/examining PDF's from the file manager. Calibre, on the other hand, is what I use to organize my e-books. I even tried using it to synchronize my collection from my desktop to my phone (though that didn't go well, and I ended up just using Syncthing for that).