Does not work. HTTrack does not do AJAX-based POST requests.

You are not seeing any downloaded PDFs, because the webpage you provided does
not list any PDF links.
These are going to be only displayed once you change for example the
pagination size or issue a search request with the provided functionality on
that website.

Even if the initial website call would list the first 12 pattern PDFs, you
would then latest hit the issue of pagination, which is in place in order to
reach out to the remaining 3031 patterns.

Short: manual interaction is required on that website in order to even start
seeing PDF links. And such things are not possible with HTTrack.

p.s. keep in mind this is a volunteer-driven community. If you urgently need a
solution consider to hire a developer. Your scenario is pretty simple for a
custom solution and should be feasible within 2-3 hours for an
intermediate-level developer.