Hmm this is rather strange :/ What are you saving your presentation as Tom? This might give us an idea what is going on.
Hmm this is really strange then because that normally should work. Maybe this is a time to use the group blog to your advantage and ask on there?
I'd consider shrinking the size of your resulting PDF using Acrobat before uploading to Scribd maybe?
I don't have Acrobat. :(