Overall, Adobe Digital Editions is a great application to view and annotate PDF and EPUB documents. The application itself is fairly lightweight and consumes little system resources. The user interface is standard, but well organized and is easy to use. There are a few cool features that are worth mentioning with Adobe Digital Editions such as the ability to 'borrow' digital content from a public library; the process remains as it would to purchase digital content, but without the payment process.
The annotation and highlighting functions work well too. So, if you need a basic EPUB and PDF reader/manager then Adobe Digital Editions is a great free tool.