Microsoft is introducing ads into a free version of Office, which seemingly only lets users save to OneDrive.

In other news, LibreOffice is also free, doesn't have ads, and lets you save your documents wherever you want. I use it every day for my work. It is available here:

https://www.libreoffice.org/

Home | LibreOffice - Free and private office suite - Based on OpenOffice - Compatible with Microsoft

Free office suite – the evolution of OpenOffice. Compatible with Microsoft .doc, .docx, .xls, .xlsx, .ppt, .pptx. Updated regularly, community powered.

@neil I would also add that if you decided to stick with an older version of Office for whatever reason, LibreOffice also opens more recent versions of Excel and Word docs.
@servelan
And still you get massive formatting issues, if you work on these documents with MS Office and LibreOffice.
MS Office even destroys The formatting of Open Document files, but does The same with e.g. .docx files or files with Macros like .docm .
I test this regularly and it's a pain.
Just with AutoCAD and converting to free formats and back is a pain.
Sadly.

Personally it's LibreOffice or aonlyOffice for me, but at work it's MS Office, sadly.
@neil