Some PDF document files are protected by the creator by disabling specific functions on the PDF reader. A common restriction is to prevent copying or editing the document to protect the copyrights of the owner. One way to copy such PDF files is to take a screenshot and use an OCR tool to convert it to text. An easier way is to use a PDF unlocking program.
PDF Unlocker is a free application that can remove the restrictions set by the creator. Simply drag the PDF file to the shortcut of application and it will automatically create a non-protected copy of the PDF file. (Thanks GHacks)
If you want an online solution, then try this free PDF unlock utility. The maximum size of the uploaded PDF file should be less than 5MB.