A site 

www.example.com/filepage/files.html 

has a bunch of links to PDF files that we need, which are stored in 

www.example.com/xyz/pdf/

and we want to download those PDF files.  The files.html page also
unfortunately has a link to 

www.example.com

so httrack happily starts downloading everything from there.  We don't need
that.

I have tried various permutations of the options.  The GAQ doesn't help much:
It says "Example : httrack www.someweb.com/someaddress.html +*.zip" will allow
you to retrieve all zip files that are linked on the site.

That seems to be documentation for a command-line version of httrack.  If I
only want PDF files from www.example.com/xyz/pdf/ that are linked from
www.example.com/filepage/files.html, what should I do?  Thanks.