Command-line Decrypting of PDF, given a document open password

Yes. Qpdf should do what you want:

qpdf --password=thepassword --decrypt input.pdf output.pdf

You can find a download for Windows (assuming you have MinGW installed) if you go to Files->qpdf->latest version, then scroll down to find a description of the files (you can find a link to version 5.11, the most recent version as of 2014-3-28, here).

Choco users can install the package by issuing choco install qpdf (ref)