Can I back up all the microsoft files on my PC with icloud?

One option is to install iCloud for Windows on your PC and then save your files to iCloud Drive as described in Set up and use iCloud for Windows - Apple Support

Another option would be to use Microsoft's OneDrive. They offer 5GB free storage or if you subscribe to Microsoft 365 it includes 1TB of storage. https://www.microsoft.com/en-us/microsoft-365/onedrive/online-cloud-storage

Just downloading iCloud for Windows doesn't accomplish it. You would also need to turn on iCloud Drive and then save or move files to the iCloud Drive folder. See the instructions in the two links previously provided.