Since the PDF library is not free software, it isn’t included with the distribution of Chromium. To get PDF support in Chromium:

Download the appropriate DEB package for your system from the google chrome download page. Using the archive manager, extract libpdf.so from /opt/google/chrome/. I extracted the file to my desktop. Copy the library into your chromium installation libs:

sudo cp ~/Desktop/libpdf.so /usr/lib/chromium-browser

Restart Chromium if you have it open, and verify that the Chrome PDF Viewer is enabled via about:plugins in the URL bar.

Source: http://askubuntu.com/questions/12584/why-doesnt-chromium-have-chrome-pdf-viewer-plugin