I just downloaded latest LO version. When I attempted to open a Calc file saved as xls, I saw it had been converted to a pdf file. I right clicked on it and LO Calc appeared as an “Open with” option. I was then able to open it but when I did a ‘Save As’ xls, it saved it as a pdf file. This appears to be a bug associated with the LO download.
The first thing to do when using Windows Explorer: Remove the option to not show known file name extensions - and you won’t claim ever again an application changed your documents to some other format automatically.
The files have not been converted. The default program to open them has been changed from Calc to your pdf reader; this might have had a different cause than suggested.
“Open With”" makes me think you might have Windows 10.
If you do then Right click on an xls file, select Open With > Choose another App.
In the dialog that opens tick the box that says Always use this App to open xls files then click LibreOffice Calc (click OK if dialog hasn’t already accepted your input and closed).
You should see the icon for xls file change from PDF reader to a Calc icon