For those of you who have tried to annotate a pdf file, it turns out te be a real nightmare. Yes Adobe Acrobat reader do allow you to do, but there is a little but...the pdf file must be configured to do so. If you take a look to the pdf properties you will see the following:

Can you remove this restriction? Yes, if you purchase Adobe Acrobat you can save the file with the required flag properly set-up. It's quite expensive for annotating a few pdf files don't you think? Well after a few trials I have found PDX-Xchange viewer. It will not only allow to place sticky notes here and there but also stamps such as "Confidential", "Draft",...etc. And best of all, it's free.

 Just visit:  http://www.docu-track.com/home/prod_user/pdfx_viewer/

Update: You may also want to try Nuance latest reader which seems to provide similar features also for free: http://www.nuance.com/imaging/products/pdf-reader.asp