As far as I can tell, MarginNote is the only iPhone app that lets you annotate & highlight both PDFs and epub files, and sync those annotations to your computer. And by “PDFs and epub files” I basically mean “all text files,” since Calibre and other apps can convert any text file into an epub, except for PDFs with tables and images. (The Kindle iPhone app can annotate text files, but can’t sync those annotations anywhere unless you bought the text directly from Amazon.)
This is important for people who like to read nonfiction “on the go,” like me — and plausibly some of my readers, so I figured I’d share my discovery.