DocSend support for Excel spreadsheets (.xls and .xlsx) is currently in Beta.
Every once in a while, if a spreadsheet is particularly complex and interactive, we will have difficulties processing it. If you're having difficulties uploading a spreadsheet, we suggest you save as a PDF and then upload to DocSend. We find that uploading as a PDF often fixes any issues you may be experiencing.
If you have a spreadsheet that fails processing and you feel comfortable shooting it our way, please send it over to firstname.lastname@example.org, so we can continue to gather examples and improve the way we handle spreadsheets.
We'll be sure to keep the Help Center updated when Excel support comes out of beta in the future!