Search engines definitely crawl PDF's. I don't know if they follow PDF links back to websites, but your users definitely can!
One important thing to keep in mind: If the PDF doesn't have embedded text (ie, the PDF is just a large image), the bots won't be able to read the text. For example, if you create a large flyer in Photoshop as a PSD and print it to PDF, the text is going to be treated as an image. But, I use an older version of Photoshop (CS), and maybe newer versions will create PDF's will actual embedded text instead of a flat image.