Update:
If you have know the password, and want to remove the password, please use this free software,PdfCrypt
PDFCrack source code has been released over a month, but there is still no windows binary version, so I decide to do it, and it come to the world.
http://blog.rubypdf.com/pdfcrack/
I do noting, but use Cygwin to compile PDFCrack under Windows [...]