How to Work with Excel Web App Workbooks
Creating a new Excel Web App document in a SharePoint Online document library is easy. On the Ribbon, click the Document tab and then click the New Document button.
The New Document button is used to create a new document. The type of document that is created depends on the content type that is set as the default for the library. When you create a new document library, you can choose the type of content it will hold.
If you choose Excel Spreadsheets as the type of document, then clicking the New Document button will create a new Excel document. If you choose Word Document, then clicking the New Document button will create a new Word document.
When you create a new document, SharePoint is smart enough to determine if you already have Microsoft Office Excel installed locally on your computer. If you do have Excel installed locally, then Excel will open up so that you can develop your document in the full-featured application.
If you do not have Excel installed, then the new Excel document will open in the browser in Editing Mode so that you can develop your document by using the Excel Web App.
After you have finished developing your spreadsheet, you can save it, which will automatically save it to the document library in which you created it. You can then click on the document to view it and then edit it further by using either the Excel Web App or the local Excel application running on your computer.
There are times when you do not need to edit a document. For example, you might want to just view the latest spreadsheet report or show a colleague a set of data. The Excel Web App contains two different modes. When you are editing the document you are using Editing Mode.
When you want to read the document, you can simply click it in SharePoint to open it and view it. This reading view is called Reading Mode, which looks very similar to a document that is printed on paper.