The best alternatives to Secretmoji are Timebomb Messenger, SafeNotes, and Flamyngo. If these 3 options don't work for you, we've listed a few more alternatives below.
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Tanka— Tanka = Slack + Glean, built for fast-growing startups.
Easily annotate documents directly while reading them with real-time synchronization. No one but you has access to your PDF and annotations. All your PDF files & annotations are stored encrypted with keys only you control.