Hi,
This is what I see when I upload a PDF file (using ACF front-end form)
http://i67.tinypic.com/28v3lus.png
It looks like a bug. Of course, I’m not expecting to see a PDF preview, I’m expecting to see a generic file icon like this: http://i66.tinypic.com/549mkg.png
I see a missing image icon instead.
I’ve marked this topic as a bug report, but I’m not sure is it an ACF bug or a WordPress bug.