After made iText worked on Google App Engine, I built Another application, Remove PDF Password Online. It bases on iText 2.1.7 but with many modification.

the main features of pdfdecrypt online are,

• remove restrictions on any secured PDF document (you should have the right to do it, for example, if you forgot the password). Any Acrobat version up to 9 is supported, even with 128-bit AES or 128-bit RC4 encryption. PDF restrictions removal is an instant process. Unlocked file can be opened in any PDF viewer without any restrictions so you may edit/copy/print it.
• remove the PDF open password. Decryption of the file with password for opening is guaranteed for PDF files Any Acrobat version up to 9 is supported, even with 128-bit AES or 128-bit RC4 encryption,but you must know the password first.

It supports upload PDF documents from your disk and give a PDF url.

If you like it, just have a try now, it is free.

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 XP
If you want source code, please download from pdfcrack project page.

Features available in this release (check TODO for features that might come):
* Both owner- and user-passwords with the Standard Security Handler, rev 2 & 3.
* Search by wordlist
* Search by bruteforcing with specific charset
* Optimized search for owner-password when user-password is known (or empty)
* Extremely simple permutations of passwords (makes first letter uppercase)

- currently only useful for bruteforcing with charsets:
* Auto-save when interrupted (Ctrl-C or send SIGINT to the process)
* Loading saved state

- currently only for bruteforcing with charsets:
* Minimum length of password to start at
* Maximum length of password to try

Sort your wordlist by length for best performance and consider that almost
all passwords in PDFs are in iso latin 1 so use the correct character encoding
in your terminal and/or wordlist when using special characters.

This tool can not decrypt a Password Protected PDF.
Look up the pdftk toolkit which can do that, when you know the password.

References:
http://sourceforge.net/projects/pdfcrack/
http://www.cygwin.com/
http://blog.rubypdf.com/pdfcrack/
PDFCrack – A Password Recovery Tool for PDF-files