Version:            (using KDE Devel)
Installed from:    Compiled sources
OS:                Linux

It seems as if kde uses the extension of a file to determine it's mime type.
When you have a pdf which is called foo.cgi e.g. (in my case created on the fly by a broken webpage) kmimetypefinder reports this as 
| application/x-cgi
| (accuracy 100)
If I use 'file' I get the following:
| PDF document, version 1.4
Because of the wrong mime type the pdf can't be opened by okular.
The interesting thing: pdfs with a non-standard extension (.fweeg or whatever) are correctly classified as pdf.

How to reproduce:
Take a pdf file with .pdf extension, run kmimetypefinder on it -> application/pdf is reported
Change the extension to .cgi, run kmimetypefinder on it -> application/x-cgi is reported
The same happens with .txt or .wav, or...