How to send an encrypted folder to a windows 10 user

If you can create a PDF document of whatever file you want to send, then Preview can encrypt that. I just created a screenshot and then chose to export as a PDF in Preview:

You would then send the recipient the password once (via email or any other way you wish) and then attach the encrypted pdf to another email. As long as the recipient can open/read pdf's, all they'll need is the password.

If you are looking for a good way to transfer encrypted directories between MacOS and Windows 10, you will need to use a 3rd party application. Personally, I continue to use TrueCrypt, which in spite of the fact that support was dropped for it a few years ago, continues to be one of the best ones out there.

You'll have to do some modifications on the install file for MacOS if you want it to install on Sierra though.

If this was a Mac to Mac requirement then using Disk Images would be the logical solution. If it was Windows to Windows then possibly BitLocker which is Microsoft's equivalent would be the approach.

Because you are going Mac to Windows and possibly the reverse your options are much more limited. Both Mac and Windows support encrypted ZIP files but as you say this is not the best in terms of security. I also would initially have suggested TrueCrypt but being discontinued any security weaknesses it may have will not be fixed.

There is one other software solution that is worth considering which is or was PGP Desktop. This is available for Mac and Windows. PGP is now owned by Symantec, unfortunately like most such ghastly giant software corporations they have rebadged it and merged it with other products such that it is hard now to tell which current product does the equivalent job. Symantec, McAfee, Oracle, Computer Associates they are all as bad as each other. 😕

I think this is the correct webpage - Symantec Endpoint Encryption | Symantec

Note: You would not want to use the full-disk encryption option which is equivalent to Apple's FileVault2, you only want to use the removable media encryption.

An alternative approach is to use a hardware encrypted USB memory stick, this comes with Mac and Windows apps to 'unlock' it and is what we use as a law firm. We use these http://www.kingston.com/en/usb/personal_business/dtlpg3

Well, I am not a security expert; neither am I (nor anyone else here) privy to Apple's internal information - and they certainly would probably not share any details regarding security. They never do with their security updates.

Having said that, my personal opinion is that absolutely nothing is safe online. And that would include anything encrypted although it would take a hacker longer to decipher. Unless you work for/with one of the three letter agencies and have their resources available, any consumer level encryption can most likely be hacked.

So, I heed that and try not to leave or email any info or publish/post anything I would not want to share with the world (so to speak). And, I do not envy anyone who is required to do that.

I know that you want "no additional software" but I don't think that there is any native cross-platform solutions other than an encrypted PDF.

A far better and proper option would be to use something like 1Password for Teams (https://1password.com/teams/).