PDF documents can be protected by the creator by disabling some functions such prevent printing, copying or editing the documents. Thats the reason why most users choose PDF format to share and transfer documents. They need to keep their content from being copied or stolen.
There are several ways to remove this PDF protection. You can use online or offline ways.
1. Online Ways
I found two good sites that offering a service to unlock your PDF files, PDF Cracker and PDF Filler . Both sites will ask you to upload your PDF files and all you do is just wait them finish their jobs.