[Typo3] a question about the content type

[zsl vincent]

Hallo
     i insert a new page content element of "Image" type into my 
site,and you can also see "GIF JPG JPEG TIF BMP PCX TGA PNG PDF AI" in 
the Images attribute,that means we can choose the file of these 
types,and when i choose a gif file,that goes very well,but when i choose 
a PDF file for this attribute,why it does not display in the frontend 
just like the GIF
,do i need a extension to solve this problem or is it because of the 
GDlibrary?
Thanks for any help