I supppse you want to download the pdf. I think you have not to consider that
(instruction?). You have to find the .pdf file link/path, then add it to the
Httrack url or filter before starting the download.

To find the pdf file path, if it is not visible in the browser bar when
hovering on the image with the mouse, you can search it visualizing the html
source of the page the image/link lies.