What term refers to a letter that is not always originally intended for public distribution but can become public due to the sender's or recipient's fame?

The correct term in this context is "epistle." An epistle typically refers to a formal letter, often associated with literary or religious texts, where the content is sometimes intended for private communication but can gain public interest due to the prominence of the individuals involved—such as well-known public figures or authors. This genre of correspondence often carries deeper meanings or insights that resonate with a wider audience, making them subject to public distribution over time.

In contrast, a "memo" is usually a brief and informal document meant for internal communication within an organization, lacking the same literary or public significance. A "document" is a general term that could apply to any written material, empty of the particular connotations associated with letters and epistles. A "note" refers to a brief written message that usually carries less formality and is often used for quick communication rather than the more substantial and enduring nature of an epistle. Thus, the distinction of public relevance tied to the formality and nature of an epistle sets it apart in this scenario.