> Is it possible to simply grab all files in a web site 
> directory under the following circumstances.
> - Site requires SSL connection https://

Yes. (native since 3.20)

> - Files are .pdf files

Yes. (you may have to use scan rules, such as +*.pdf)

> - there are no links to these files on the site

No :(
Httrack must have an entry point somewhere, or it won't work