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The-Vault
A 'Vault' to conatin and password-protect your most precious files. Uses AES encrytion and data URL's to give a sort of 'cloud' feel. All without a server, and totally secure!
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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
I'm not sure if it could be called a "PDF viewer" per-say, but I have a repo from a while back: https://github.com/flancast90/the-vault . It allows encryption via password protect on a case-by-case basis for each file.
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.