Thumbnails of specific file types can be disabled using dconf editor.
Install dconf editor:
Hit Alt+Ctrl+T and run the following command:
sudo apt-get install dconf-tools
Or search for "dconf-tools" in Software Center and install it from there.
How to disable thumbnails of specific file type?
Once installed, Hit super key (windows key) to open Unity dash and search for "dconf editor" and open it.
org > gnome > desktop > thumbnailer
Paste the MIME type of PDF
'application/pdf' in the value field of disable key, Like shown below.
This will prevent Nautilus from generating thumbnails of PDF's in the future but old thumbnails will be there. To solve this you'll have to clear Nautilus thumbnail cache.
To disable thumbnails for other file types, Add there MIME type in disable list as described above. Seprate multiple types with comma. like this:
For complete list of MIME types visit iana.org.