How to print Excel sheeet

can sheet (view) save in files, mail and a lot more - but cannot print them out ?

Yes, because Excel sheets can only be viewed via Quick Look — DEVONthink doesn’t include a full Excel renderer. And so formatting Excel sheets for printing is not possible at the moment. Best to use Excel for this.

Are you referring to actual Excel files or sheet documents in DEVONthink To Go?
A sheet document can be printed via the Share > Print controls though you’d likely have to modfy the print settings to get things to fit on the page.

Excel documents cannot be printed from within our application, as has already been noted.

Excel workbooks

Not at all sure what your latest question is, but the solution to print Excel workbooks is best done using Microsoft Excel.