The Open With caution message.
We've discussed opening PST files. One big difference between PST files and other types of files: You can't open a PST file by double-clicking it. If you try, you'll see a message similar to the one in the picture.
The caution message suggesting that you click Open With is the kind of result that can be frustrating, confusing, or even alarming. To avoid it, just remember that to open a PST, you always start inside Outlook and use the File menu as we described in the previous section.