PDF thumbnail and viewer

Probably a silly question but, i have a file upload field that allows .jpg,.png,.gif,.pdf

When i upload any of the image file types it shows a small thumbnail that i have set and when you click on it i have made it show in a lightbox, all working great, except for a PDF, no thumbnail and no view in the lightbox either.
Am i doing something wrong or is this normal that it can not show a pdf file?