The problem seems to be related to the way plone functions. The pdf document is
really located at
<http://sgs-muenster.net/test-httrack/Busfahrplan.pdf/Busfahrplan.pdf> (i.e.
inside the directory) while the link to the "file" HTTrack encounters is
really a link to the directory
<http://sgs-muenster.net/test-httrack/Busfahrplan.pdf/>. 

From the hts-log.txt:  

11:20:33	Info: 	engine: transfer-status: link added:
sgs-muenster.net/test-httrack/Busfahrplan.pdf ->
/Users/bt/071107_sgs-muenster_net_test/sgs-muenster.net/test-httrack/Busfahrplan.pdf

11:20:33	Error: Unable to save file
/Users/bt/071107_sgs-muenster_net_test/sgs-muenster.net/test-httrack/Busfahrplan.pdf
: Is a directory

I have not found any solution to the problem, but perhaps someone else can...


-Bo