Dropbox remains the best app for seamlessly syncing files to and from your portable devices. It works across the web, Windows, macOS, iOS, and of course Android, which means you need never find yourself without access to an important file.
You can use Dropbox to automatically back up your photos and videos to the cloud, keep that important set of PDFs with you, or take notes that sync back to your laptop.
On the negative side, the free Dropbox plan only offers a miserly 2GB of storage. However, if you need more space, premium plans start at $10 per month for 1TB of storage.