The Chrome build from Google has a Chrome PDF Viewer plugin which is really nice. But, the Chromium builds don't seem to have it (nothing shows up in
How do I get the plugin in Chromium on Ubuntu?
Because the PDF plugin is not free software, i.e. it can only be included with the non-free Google Chrome browser.
See http://code.google.com/p/chromium/issues/detail?id=50852#c16 for a statement.
tiax is correct but it can be pulled over from a Chrome install and used from within Chromium.
You just need to pull the libpdf.so file over and enable it in about:plugins
You have two solutions that I can see here (applied to Ubuntu 11.10) [This was inspired by comments above and other sources I found]
Here's how I did that...
Note: The new plugins never showed up in my extension listing but suddenly just started working properly. By copy all *.so you are getting support for Flash, FFMPEG, and PDF support.
If you want to copy each individual item here is a listing of them for you to hand pick.