Why is PDF so popular for online publishing?
Some pros and cons about it (a cons actually might be a pros, depending on the perspective).
+ points:
Portability.
Allows protection of intellectual property (IP).
Support for publishing scientific media (mathematical notations etc.).
Efficient packaging of content (less memory and high quality).
- points:
Difficult to edit and add comments without installing Acrobat Professional.
Difficult to Copy & Paste 
Difficult for collaborative editing and publishing. It is more like an end product rather than a material to be worked upon. Probably, PDF wasn’t meant to be for this purpose at all.
References:
http://weblog.404creative.com/index.php/2007/02/24/a-few-reasons-not-to-send-files-in-micorosft-word-format/
http://wordprocessing.about.com/od/choosingsoftware/a/pdfsharing.htm
http://www.afb.org/afbpress/pub.asp?DocID=aw060604
http://www.its.monash.edu.au/staff/web/slideshows/accessibility-pdfformat/