As long as "Show thumbnail" is disabled in option you will see an empty thumbnail for PDF files. Instead, I'd expect the Acrobat PDF symbol to be shown.
To reproduce (you will need AFPL Ghostscript installed on your computer):
1.) Disable the setting "Show as thumbnail" fpr AI, PS, PDF files in Tools > Options > Browser > File List > Items displayed
2. ) Close options and browse a folder with a PDF file in it.
--> XnView displays an empty thumbnail for the PDF file.

V 1.90 B3