SharePoint Foundation 2010 with Search Server 2010 Express. It's free. If you've only seen 2007, you need to check out the new stuff in 2010. You get out of the box managed metadata fields for item tagging and this integrates with the search so that you can offer a refinement panel for users to find specific categories of items.
It meets all of the requirements you have and then some. You'll have to install a PDF iFilter to enable searching the contents of PDFs, but that's really easy:
http://www.sharepointsharon.com/2010/03/sharepoint-2010-and-adobe-pdf/