There are two ways. One is using filters, for example like this:
-*
+site.com/*
-*.pdf

That tells httrack to not download anything, but all from that site, except
pdf.

Another way is like I do with wikipedia and wikihow for downloading all links
from templates, lists, tables, categories. Use firefox or chrome with
extension "copy selected links" then depth level to 2 in httrack. Then sort
manually those folders for example like I do in folder by the name of
template: Template_biology