Free PDF Download of PMBOK.

Free download of PMBOK is available on the following link.  If you want to take PMP exam my advice, buy the book.  You have to be member of PMI to access it.  This is the new PMBOK Guide 4th Edition, read only pdf file.  Technically it is not free as you have to pay for membership. 

http://www.pmi.org/~/media/PDF/Publications/Key-Messages-to-Access-PMBOK.ashx

Technical downloading instructions
To download the PMBOK® Guide—Fourth Edition, it is recommended that your computer have Adobe® Reader®, version 7 or later, or Adobe® Acrobat® Professional, version 7 or later.
If you don't have either of these, you can download the latest version of Adobe® Reader® at http://get.adobe.com/reader/. A link to this download page is also available from the Library of PMI Global Standards: Projects page:

You do not need a plug-in in order to download a PDF of the PMBOK® Guide.
When you download or print the PMBOK® Guide from PMI.org, it is marked with your username and member ID, and it is secured with your PMI.org password. This means the file is licensed to you for your personal use only.
If you print the file, it will be identified with a notice that says, “Licensed to (first name, last name, PMI member ID). This copy is a PMI Member Benefit, not for distribution, sale or reproduction.” The file is easily opened during and after download using your PMI.org password.

 You can buy on Amazon: Search Amazon.com for pmbok 4th edition 2010.  If you are planning to take PMP exam then go and buy it.