that's from the pdf section. So you can edit that if you want to point to some other type of icon.
I would like to change the icon but don`t know exactly which file to edit (/etc/xfce? /usr/share/?) and change it to what instead of"x-office-document" ?
You should change that file that you're quoting and change that line to point to an icon that is included in your current theme. Run the "Rebuild icon cache" tool when you're done with it.
In /usr/share/mime there's a file "generic-icons".
I edit that to: application/pdf:application-pdf(or your icons name)
Icon name can be found in ~./icons/yourtheme/mimetypes/48 or system /usr/share/icons/your theme/mimetypes/48
Rebuild icon cache as gapan says: -