The text and links inside PDF documents are indexed by Google (just like HTML files) and often rank high in search results so PDF files may be a good source of traffic to your website. Google OCR can read text inside images as well.
Susan Esparza posts a summary of suggestions made by George Aspland on how to use Adobe PDF files to get more traffic to your blog. Excerpts:
To get your PDF indexed in Google simply link to it from one or more pages that already indexed.
For better PDF rankings: Pay special attention to the headline in a PDF as Google always includes snippets from the PDF and puts the words and phrases of the query in bold. Also make sure that you PDF has a proper Document Title similar to the Title tag on a Web Page.