A good PDF/document manager/viewer - with tags!

I'm pretty surprised that there doesn't seem to be an excellent app on the iPad for managing, viewing and marking up PDFs and other documents. There are obviously a number of apps out there, from academically-oriented reference managers to ebook readers. Readdle's PDF Expert seems to be one of the best, but it doesn't support tags or things like smart lists. I'm basically looking for this - an app that can display the documents as thumbnails on a grid, and which also supports (Mavericks) tags. Basically a version of Yep for the iPad (unfortunately Ironic Software don't seem to be making one, which is a shame). Does anyone know if anything available like this? Seems strange that there isn't something available for something this basic and presumably in demand...