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 about:plugins).
How do I get the plugin in Chromium on Ubuntu?
Chromium – Why Doesn’t Chromium Have ‘Chrome PDF Viewer’ Plugin?
chromiumpdfplugins
Best Answer
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.