If you delete a file from the web interface, it's deleted from your computer. So if you change a file from your phone, that file is saved on the server and synced down to your computer with those changes. The biggest risk you run, however, is inherent to Dropbox itself: any changes made to a file in Dropbox are synced to all devices linked to that account. If you change a file on your phone or with Dropbox on the web, that file will be synced down to your computer, and vice versa. Make sure you have a strong and unique Dropbox password and use pin or Touch ID security in the mobile apps when available. There are risks that come with putting your files into the cloud, of course, and they come down to you to manage them. Chances are you'd be able to back up your entire computer onto that if you really wanted to. Thankfully, Dropbox isn't terribly expensive, offering 1TB of storage for $9.99/month. That might be enough for you, but chances are if you're planning on putting an entire folder like Documents in there you'll need more space. The first is cost: Dropbox does offer a free tier, and it's a paltry 2GB. Heck, you could even sync and back up your Mac's desktop files and installed apps. You can put your Pictures folder into Dropbox with a symlink or your entire Documents folder. The additional benefit is that you can sync and back up entire directories that would be otherwise difficult to move. So with one 25-byte symlink you can back up gigabytes upon gigabytes of data without duplicating it on your computer. But if you put a symlink into the Dropbox folder, when the Dropbox backup app looks at it it's redirected to the linked file, and it backs that up instead. If you were to create an alias to a file and put that in the Dropbox folder, only the alias would get backed up, and that's not terribly useful. Put a tiny symlink into Dropbox and it can back up an entire folder elsewhere on your computer Thing is, if you want to back up files to Dropbox, copying them into that folder means they're taking up space twice on your hard drive, and just moving them to Dropbox takes them right out of your organizational flow. Upload a file from a Dropbox app or their web interface and it'll be downloaded onto your computer. Place a file in that folder and it's synced to the cloud.

Dropbox works by creating a folder on your computer that is synced with their servers.

Dropbox kp2a folder