There are two Confluence add-ons for securing content:
Both of these plugins allow you to store encrypted content on a confluence page.
Attaching a document and having that document encrypted is a bit different and does not seem to be a feature of either of those plugins. However, there's nothing stopping you from using a third-party tool to encrypt the document client-side, upload the encrypted document it as a regular attachment, and then storing the encryption key in Confluence (perhaps even in one of those macros).
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