I used Libre Office (LO) and whilst it can open existing Excel files it loses all the Excel formatting and cell contents became simple alpha-numeric characters. For example suppose an Excel cell "A2" contained the formula "=B3+B5" and the answer in "A2" was "8" then, in LO, the formula was replaced by the character "8" and so any changes to cells B3 or B5 did not affect cell "A2" which they should have done. LO is probably good for new spreadsheets but not for working with existing Excel files. I had a few Excel files so I was forced to go back to MS Office.
Well, its free so worth a try. I have had no problem using it to open and work on existing files created in Excel. The only thing I find is that you still have to learn where some of the Menu items are but all the main ones are pretty obvious.
When you install do the main Program first THEN the Help Package. During the first one opt for the Custom Install NOT Typical. You associate everything with the MS Office equivalents and choose .XML formats which are the latest MS formats.