I have a menu system and it reads all the files in a particular folder. These are typically linking documents to other pages within my site. However, I now need to add a PDF file to this menu system, so I used the link to existing document and chose the particular file i wanted (its a file document type). But, the file will not show up in my menu (I am using the CSS List Menu webpart) yet other linked documents (that are page menu items) will show up.

Any idea why this might be happening?