Ask HN: The safest way to store data online?

Imagine you have to build the safest way to store data online.

As "data" in this example, we take something really simple : a PDF.

And by saying "safest", I mean the highest level of security + usability/ease of use.

1. Which technology/ies would you use to ensure your users they deal with the safest storage solution online ?

2. And if you allow your users to share this PDF with someone, would this choice change ? You have to ensure the same security by sharing the document/data.

(FYI : we already built a solution - I want to compare our solutions)