My client wanted that option too, but the only way was to zip it first, which is daunting for the average downloader. My answer was to use Zoho Sheets, and embed it in the webpage and allow the spreadsheet to be used online, and still have the option to allow user to download the sheet. See here: http://gpstrackusa.com/roi.htm (my client asked me to disable the download option, but it would be right next to the "click to edit" button.