The idea behind Yep is simple: iPhoto helps you with your pictures and iTunes handles your music--but what do you use to work with your PDFs? Yep is a dedicated document manager, almost a "specialized Finder" just for PDFs. Don't have a lot of PDFs? If you're really ambitious, Yep also gives you the tools to scan and then manage all of your paper documents--moving you out of folders and filing cabinets in the same way that iPhoto gets your pictures out of shoeboxes.
Yep can handle tens of thousands of files, giving you very flexible and intuitive tagging tools (the "Yep Tag Cloud") that help you sort, view, organize, and archive your documents. You can also use Yep to convert documents in other formats (like Word files) into PDFs--and Yep doesn't duplicate or move your files, nor does it lock you into using Yep in the future with some proprietary format. Recent updates have added a lot of small refinements that make tagging, scanning, and filing PDFs even easier and more reliable.