The following forum post may be of some interest to you:
https://groups.google.com/d/msg/pdfnet-sdk/dJpyhULyYUA/0ozIje6-m5kJ
https://groups.google.com/d/msg/pdfnet-sdk/8N0DSeIpjRg/RjJG454oEfsJ
In short, while there is no password expiration mechanism built into the PDF format, you could customize PDFNet viewer code to enforce document expiration. This is kind of DRM is easiest with WebViewer (http://www.pdftron.com/webviewer/index.html, http://www.pdftron.com/webviewer/showcase/).