Yes, you're correct that the choice between "los documentos" and "mis documentos" in Castilian Spanish often depends on the context in which you're speaking or writing.

When you say "voy a preparar los documentos", it implies that you are referring to a specific set of documents that are understood in the context of the conversation. It suggests that these documents are known to both the speaker and the listener, or that they are generally defined by the situation.

On the other hand, "voy a preparar mis documentos" specifies that the documents are yours personally. This phrase is used when the speaker wants to emphasize ownership or a personal relationship with the documents.

In professional or formal settings where the ownership of the documents is not the focus, or where the documents are not personally tied to the speaker, it is indeed more common to say "voy a preparar los documentos". This usage is neutral and focuses on the task rather than who owns the documents.

Therefore, in general communications about documents that do not necessarily belong to you personally or when the ownership is irrelevant to the current discussion, "los documentos" is the preferred choice.

Un saludo

Silvia