[jem]

I found this somewhere many moons ago:

Not sure if you want a way to turn off displaying PDF documents in OmniWeb completely but if you like you can do so by opening a terminal window and entering this command:

defaults write com.omnigroup.OmniWeb5 WebKitOmitPDFSupport -bool YES

After doing so, restart OmniWeb and PDFs will always download (A preference for this would be handy).