A family of Microsoft word processing software products for creating web, email, and print documents.
Thanks for the update.
If you are prompted to use the Save As feature after using the hotkeys Ctrl + S. It is most likely that the file is not yet saved locally on your computer. E.g. You open an Excel spreadsheet that is attached on an email. Excel preloads the file letting you edit it now and decide where to save it later. If the file is already saved locally and you edit it, pressing Ctrl + S will save your file without having the hassle to go to File then Save.
In regard with signing in using with your Microsoft Account in Office. Normally you don't need to be signed in just to save your files locally. By signing in with your Microsoft Account, you can connect your Office applications to other Microsoft services such as Office Online, OneDrive, Skype, etc.