Digital Signature using certificates

Our company is  thinking about implementing digital signatures in Adobe Reader using our internal certficate authority where users have a certifcate that can be used for email encryption and signature but we do have a couple organisational questions 

1. What is if somebody signs a document with a certficate that is valid for 3 years and after 4 years , so after the certficate expires somebody looks at the document is the singature still there valid? 

2. What is somebody signs the documents and leaves the organisation is the siganture still valid/good? 

Thank you for your help

Christian