CryptSync is simple and easy to use tool to synchronize 2 folders, where the files in the destination folder are encrypted. (CryptSync is using 7-Zip for the encryption.)
In this way, one can perform client side encryption when sharing several files. Each file is encrypted individually with the same password, and it is only the encrypted files that are stored in the untrusted media (see figure below). It is possible to use CryptSync in a portable way by copying the files without installation on the second computer.
Depending on the use case, like which client operating systems need to be supported and which media is used to sharing, then there are several other alternatives like Cryptomator and Syncany. (It is possible to use cryptomator without installation, with jar file, if using JPortable from portableapps.). However, they are a little more complex than the CryptSync. One also could reuse the same idea, and DIY (do it yourself) with a scripting tool, like autoit…
For sharing one file at at time over untrusted media or via email, then one could directly use 7-Zip or tool like AxCrypt, which is nicely integrated with Explorer (there exist a portable version as well AxCrypt2Go.exe).