Do you use the latest Microsoft Office software on your computer? Then you would have found that all the files which you had created in the MS Word or MS Excel applications are stored with a .XLSs and .Docx extension. Its recommended to store with these extensions and use the files in that way but well when you are sending these files across to someone else you should find that those files wont be accessible to them. The most important reason for this would be that the Microsoft Office software installed on their computer would be of a older version which is the reason they can only access to .XLS and .DOC files.
Here is a quick tool which you can use for xlsx converter – Web-service for simple conversion from xlsx to xls, pdf, odp and csv formats. Just upload the .XLSx file from your local computers hard drive and you will get access to the converted file instantly.