Can excel spreadsheet be updated with new submissions data?

Once I've started exporting PDF submissions into an excel spreadsheet, can I update the spreadsheet to add new data when I receive new PDF submissions the next day  and so forth? 

Yes, you can update your excel everyday by following this guide: https://www.jotform.com/help/44-How-to-Export-Form-Data-to-Excel. 

I suggest you try Google Spreadsheet Integration, it will update your spreadsheet automatically if you receive a submission. Here's a guide: https://www.jotform.com/help/228-How-to-Integrate-Form-with-Google-Spreadsheet

Hope this helps.