I need Mac software to keep password-protected encrypted files. I want to be able to decrypt the folder every time I need to access the files.
I imagine you want this so that even when your Filevault is unlocked, this folder is still protected.
You can just make an encrypted, self-expanding disk image [sparsebundle], entirely from within the system, using Disk Utility. You really don't need 3rd party involvement at all for encryption on a Mac.
At creation set the size to be the maximum you think you will ever require. The image on disk will only ever take up the amount of space that is actually used, not the full amount, unless you do fill it right up.
Once created, you mount it to the desktop like any other disk [simply by double-clicking it]. It will require the password at open. Once mounted it looks & behaves just like any external disk, SD card etc.
When finished you just eject it & it is again secured.