>>>Hi,
>>>
>>>I need to send someone a confidential document. Currently it is an Excel spreadsheet. I know it is impossible to make it 100% secure against all kinds of hackers. But 90% is good enough (or whatever it is). How can I encrypt this document? Should I convert it to a PDF and encrypt PDF? Or encrypting Excel is ok? I don't want to download and install any 3P tools, just whatever I have on my Windows 7. With the intent that the recipient will open it on a Mac computer.
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>>I'd work with the delivery method, rather than the document itself. For example, deliver it via a shared DropBox.
>
>I thought about it. But is Dropbox secure? That is, I can put this document in my Dropbox and give the recipient access to this folder/file. But then a hacker can hack my dropbox, no? I would prefer to do it over Dropbox but I never thought that it was very secure (maybe I am mistaken).

Is it the delivery method you want encrypted, the document itself, or both? One option is to simply put the file in a password protected .zip file with a strong password (use at least 15 characters - and I assume you know the drill on creating a decent password) -- just make sure it's at least 15 characters - only way to get to that is by brute-force attempts - and if it's long enough it would take a hacker forever to get to it. Frankly i wouldn't trust dropbox or google drive or anything else like that myself.