Unless you have been living in a cave since the early 90s you know that cyber security is a huge issue and that you should protect your data as well as you possibly can. If you are sending private information in documents and spreadsheets over the internet via email, Dropbox, iCloud or any other service, it becomes vulnerable, regardless of how secure you think the medium is. And who knows how the recipient will handle the info?
Numbers is Apple's spreadsheet application that, along with Pages and Keynote, make up the iWork suite. Chances are that the vast majority of spreadsheets you encounter at work or elsewhere are going to be Microsoft Excel files. Opening an Excel file in Numbers will change it to a Numbers file, so if you make changes to it you will probably want to save it as an Excel file if you plan to pass it back to your Excel-using co-workers. This can be easily done on your iPhone, iPad and Mac, here's how:
iPhone and iPad