Hi. I'm using Omeka.net, and this question actually doesn't even have anything to do with Omeka, I don't think, but with bugginess on Google's side.
That said, I have one item in my collection that has a pdf: http://nauck.omeka.net/items/show/11
The Docs viewer has been extremely buggy. Yesterday, when I installed the plugin, it didn't even show up. Today, the embedded viewer showed up, but for several minutes, it had a message rather than a pdf in the viewer:
"You've reached the bandwidth limit for viewing or downloading files that aren't in Google Docs format. Please try again later..."
Which I know is patently untrue. Then, ten minutes and a push of the refresh button later, it worked fine.
I'm pretty sure this is a known problem on Google's side, related to this problem with Wordpress's Docs Embedder:
...But that said, I was wondering if anyone was familiar with anything I might be able to do with the settings or anything on the Google side that might make the Viewer more reliable?