The Office app for Windows 10 is a free app that you can use with Office 2019, Office 2016, Office Online, or any Microsoft 365 subscription.
See all of your Office apps in one place and switch between them quickly. Use the Office app for Windows 10 to launch Word, Excel, and PowerPoint. Open files from OneDrive or SharePoint, or files that you have stored on your computer. Upload files to the cloud. Pin files so they're easy to get to, and see recent files, and files that others have shared with you. Use Microsoft Search to find apps, documents, sites, and people.
Anyone can use Office Online for free to create and edit files. If you've already paid for Office, you can download and install Office on up to 5 devices.
Why did www.office.com or the Office app open automatically?
The keyboard shortcut combination Ctrl+Shift+Alt+Win opens the Office app or www.office.com. Sometimes existing shortcuts, macros, and even drivers use this same keyboard shortcut combination. If you've relied on the Ctrl+Shift+Alt+Win shortcut to perform other actions in the past, you'll notice that using it launches Office now.