Disable autosave in Preview

Solution 1:

Per Turning Off Auto Saving In TextEdit And Other Mac Apps on the "Mac Tricks and Tips" website:

1. Open System Preferences
2. Go to "General"
3. Enable "Ask to keep changes when closing documents"

Solution 2:

While working with doc in Preview, hit the title bar of the doc and set its status to 'Locked'. This will lock the doc in Finder. If you'll try to edit it while it's locked, then system will propose to you to Duplicate the file so you can work with the copy.

Generally speaking they disabled this functionality in Yosemite since if you want to preserve original doc, you can easily create its copy and work with the copy. In all other cases docs are being autosaved as it is of great help to many other users.

Solution 3:

Sadly, the ApplePersistence preference no longer works in Mojave and later. The only option is to disable Auto-saving in ALL applications, in System Preferences > General. This may not be desirable.

If you really want a PDF viewer that does explicit saves (or is entirely read-only), then you need to use a different application.