Ubuntu provides a free pdf reader which cannot handle fillable form. If you attempt to open such a document, you would get the following message:

If this message is not eventually replaced by proper contents of the document, you PDF viewer may not be able to display this type of document.

You can upgrade to the latest version of Adobe Reader for Windows, Mac, or Linux by visiting http://www.adobe.com/products/acrobat/readstep2.html.

For more assistance with Adobe Reader visit http://www.adobe.com/support/products/acrreader.html.

The simplest solution to this problem is to install natty's Adobe Reader.

$ sudo apt-get install acroread