When you offer the link to the pdf (with a jpeg displayed as a thumbnail ) then you allow them (your user) to get a quick view of what the document represents, clicking on the link or image allows them to download it
(or view it richly and directly in the browser if the have the PDF plugin for their browser.
This is the best, simplest and most accessible way of present PDF and other rich media and will work on ANY browser, not just desktops, but PDA's, Iphones, cellphones and even assistance technology for the blind.
You CAN go another route, and render the pdf directly in FLASH as an SWF file.
www.pdfmenot.com | TECNERD
This works well if you dont want or cant install the adobe pdf reader.
There are also alternatives to the adobe reader as well, eg. "Foxit"