You can sync PDFs to your iOS device using iTunes and view them using the iBooks app (free, app2.me/2403). Simply drag the desired PDF from your desktop computer's file explorer to the Library panel in the upper left of the iTunes screen.
You can also view PDFs attached to e-mail or a Web page. Simply touch and hold the PDF icon and select "Open in iBooks" from the pop-up menu. This adds the PDF to your iBooks library and opens it.