I use several apps for PDF viewing and one feature that keeps me on the prowl for the holy grail of PDF viewers, is an app that supports PDF markup Being a musician and heavy e-reader, I rely heavily on PDF markup 

Dropbox, give us some simple PDF markup abilities, eg, highlight, pen, erase, opacity, and you will put several PDF viewer apps to bed Couple these features with the ability to share a folder of PDFs with other Dropbox users and you've converted me, and countless others, to PDF viewing through Dropbox