> To Download just PDF files set: 
>    Option>>Scan Rules to
>       -*
>       +*.pdf 

Beware that this filter will also exclude all html files .. 
and httrack needs them to get link to PDF files!

Something like:

-* +www.example.com/*.htm* +www.example.com/*.php* 
+www.example.com/*.asp* +*.pdf

Would be safer :)