The other main cloud storage providers both have spreadsheet editing software. And they apparently no longer support Dropbox. My only option to have a shared spreadsheet stored in the cloud with easy editing ability seems to be to use either Google sheets (with Google drive) or Microsoft Excel (with one drive).
Your PDF markup feature is fantastic. Something like that would be great. Hit the edit button and bingo you can enter some data. No downloading or opening in a 3rd party app.