I think it is not a multiple access denied. I think instead that the file may
lie in a position not following the page in which the images lie or elsewhere
outside that domain.

I think that it should be considered the httrack option 'Experts only'. If you
look at that option you'll see the two voices 'Travel mode' and 'Global travel
mode'. The default value don't permit e.g. to go up in the branch of the site
tree you are interested in. So I think you could simply to set the right value
in that options.

Otherwise you can follow my suggestion (that is what it seems you have done),
i.e. catching the path of the pdf file, and putting it in the urls you wish
(e.g. +http://thesite/.../page-of-link-images/.../image1.pdf -- supposing it's
not true my initial hypothesis, i.e. that the file is e.g. outsite the domain
or uphill in the tree structure).