I should have read your previous posts...
A PHP script --, used to verify your membership and maybe
many other parametres, knows where the pdf files are and
redirects (the files have not been moved) by using the
variable p(without p and login it will ask for it or freeze
your computer).
The pdf files may be located anywhere on any server and you
cannot guess from the fake pdf where the files are.
You could ask, as a member, in which folder they are and
then you might find scan rules and download them with
HTTRack, but you will have to link them manually.
This is not a bug, but a kind of protection anyone can use
with a scripting language (I use a script like that, but I
let any utility download the files).
What you could do, if you want to find the address is
downloading one pdf file with a download accelerator
(leechget or nettransport or ...).
If it downloads the file, it will give you the full
path(analyse or properties).
Then you could find a solution with scan rules, but I doubt
it, or link the pdf files by modifying the pages of your
mirror (a few minutes once your pdf files have been downloaded).