It's relatively easy to do, if you use Solr collections. It's been a while since I've done it, but PDFs can be keyword searchable - I've never had success with phrase searching.
Also, which browser you use will make a difference. In IE, for some reason, even with the PDF plugin will still open PDFs in Acrobat - but FireFox (with the PDF plugin) will actually open in FireFox and (if you have the collection set for it) will also highlight the keywords in the PDF document.
^_^