> The site has lots of .pdf files whcih is what we want to 
> use. In the instructions I told httrack to download .pdf 
> files but when we tried to view them offline got the 
above 
> error message.

I strongly suspect the server to have given text/html type 
for these PDF files, leading to a file corruption (that is, 
httrack tried to parse PDF files as if they were HTML 
ones). 

Do you have the bogus URL (address) please ?