I have three methods:
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For individual documents to family members, iMessage.
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Sometimes an encrypted PDF in eg Dropbox when we’ve previously agreed on a password (in discussion over FaceTime).
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For sharing files with ‘strangers’, I use 1Password’s secure sharing feature. Since you can add documents, arbitrary fields, and notes to 1Password items you can basically share anything. In the past I’ve even created a ‘note’ item and used it to write out (markdown-formatted) instructions/info and attached a file to the note. It’s a nice attractive way to provide a secure document to non-techy people. The secure sharing allows options such as limiting the recipient to a specific email address and time- and view-based limits.