Just tried to access the manuel from the free Ultima IV in my collection and I always get a message "403 - Access forbidden" from wpc.11df.edgecastcdn.net. Tried it with other games too, bought and free games, same result (Ultima, Dungeon Keeper, Dungeom Keeper 2, ...). Tried it in Firefox and IE. Still the 403 message. What am I doing wrong?

This is usually a proxy error, since GOG uses an IP based authentication. If GOG sees one IP but the CDN sees a different one, you get the 403 error.
You can try the which should be able to grab the bonus content, or if you are using linux the [url=http://www.gog.com/forum/general/lgogdownloader_gogdownloader_for_linux]lgogdownloader.

Edit: links added.

That worked, thanks a lot. Didn´t try the downloader as I usually don´t want to install such tools. (I´m also still downloading my WoW addons manually from curse.com instead of using their tool .... old-school)