Using the correct file types for SharePoint Document Libraries
In order to take full advantage SharePoint and OneDrive for Business with Office 365 applications. We recommend you save the file in Office Open XML file format. This is commonly seen as .docx for Word, xlsx for Excel and .pptx for PowerPoint.
The major advantage of doing this is to allow collaboration between multiple users. Your entire team can work on the one document and all can see the updates and comments in real-time.
Unfortunately, this does not work with Office 2003 file formats.