Do you know about the filter rules?
<https://www.httrack.com/html/filters.html>

Based on the zip file example, this might work for you:

-*
+www.example.com/*.html
+www.example.com/*.pdf
-mime:*
+mime:text/html
+mime:application/pdf

It should make httrack download only html files and pdf files.

You can set filter rules in the scan rules dialog to control which files
httrack downloads.

<http://www.httrack.com/html/step9_opt4.html>

I've not tested this.

Let us know if it works for you!