You can use the Disk Utility in macOS Sierra to create a secure disk image for confidential documents that you don’t want others to see without your permission, you can put them in an encrypted disk image. Here’s how:
- Choose File > New Image > Blank Image.
- Enter a file name for the disk image, add tags if necessary, then choose where to save it. This is the name that appears in the Finder, where you save the disk image file before opening it.
- In the Name field, enter the name for the disk image. This is the name that appears on your desktop and in the Finder sidebar, after you open the disk image.
- In the Size field, enter a size for the disk image.
- Click the Format pop-up menu, then choose the format for the disk: Mac OS Extended (Journaled) or Mac OS Extended (Case-sensitive, Journaled)
- Click the Encryption pop-up menu, then choose an encryption option.
- Enter and re-enter a password to unlock the disk image, then click Choose.
- (Note that if you forget this password, you won’t be able to open the disk image and view any of the files.)
- Use the default settings for the rest of the options:
- Click the Partitions pop-up menu, then choose Single partition - GUID Partition Map.
- Click the Image Format pop-up menu, then choose “read/write” disk image.
- Click Save, then click Done.
Disk Utility creates the disk image file where you saved it in the Finder and mounts its disk icon on your desktop and in the Finder sidebar. In the Finder, copy the documents you want to protect to the disk image.
If you want to erase the original documents so they can’t be recovered, drag them to the Trash, then choose Finder > Empty Trash.
When you’re finished using the documents on the secure disk image, be sure to eject the disk image. As long as it’s available on your desktop, anyone with access to your computer account can use the documents on it.
To access the data in a disk image, double-click it. It appears on your desktop, and you can add, remove, and edit files on it just as you would with a disk.
(This how-to is based on my experiences and info on Apple's support pages — where the images sometimes come from.)