Define the historical question
The issue may be which document version existed, which version a recipient could access, whether access changed or whether a link was replaced. The document currently reached by a link is not automatically the document available at the relevant time.
Potential evidence
Preserve the native message, the exact link and any contemporaneous downloaded copies. Authorised version history, sharing records, audit data, backups and independently held files may help where available. Record how any later copy was obtained and avoid changing permissions or overwriting the document without an agreed reason.
Scope and limitations
A mailbox export may preserve only the link. Historical versions and access records depend on the service, configuration, retention and authority. Permission to examine email does not automatically extend to all material accessible through a linked repository. Begin with the relevant date, document service, known custodians and disputed proposition. A scoped assessment can compare available versions and explain what the records support about content and access, while keeping unresolved historical questions explicit.