AES Crypt is a relatively simple program for encrypting single files. The encrypted versions can then be stored in the cloud without being readable by others, even the administrators of the storage system. An encrypted file can also be sent as an email attachment, knowing that it can't be snooped along the way. The only difficulty with securing email messages this way is that AES Crypt uses symmetric encryption, which means you'll have to inform your correspondant of the decryption password by some other means.

The program is available for several systems. You can find the download page here. Be careful using the page: When I looked at it on Windows, it had several annoying ads for a PDF converter that are designed to look like the download link, but are not.

The Windows download is a zip file which you will want to unpack. It contains a file with some instructions, but usually you will just click on AESCrypt.msi contained in the zip. The other packages seem to be a bit simpler, download and click.

As described in the Windows instructions, after installation on that system, you can right click on a file in the file explorer to produce an encrypted version, or to decrypt a file with the .aes extension. There are Mac instructions also, where a files can be dropped into AES Crypt to perform the operations.