Matthias,
I don't know of any way to send a .pdf document to a user, but if you send a .rep document the user can adjust the way they view documents within Webi to Acrobat Reader (within Webi - options > view documents > PDF Acrobat Reader).
This will then display the BO report in Webi as if it were a .pdf document and will allow the user to download it into either a thin client .pdf viewer or save it and view it locally.