With the easy ability to scan and digitally store these documents, there's really no reason to ever not maintain a digital copy of these documents on a hard drive and/or on a CD-R or DVD. As for the physical copy, if the document has expired, you can get rid of any time you want.
PLEASE READ THIS BEFORE YOU COMMENT, EMAIL ME OR PHONE ME. I'm only licensed in CA. This answer doesn't make me your lawyer, and neither do follow-up comments and/or emails and/or phone calls, and you shouldn't expect me to respond to your further questions if you haven't hired me. We need an actual agreement confirmed in writing before any attorney-client relationship is formed. This answer doesn't constitute legal advice, and shouldn't be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue.
Whose information does the agreement protect. If it is the client's, you better have them available for litigation if your company unintentionally breaches. Best of Luck!