carol I take it you are seeing the problem with all three export file types, correct? If not, make sure you try exporting as CSV. I have seen cases when there are a huge number of entries, where the Excel version of the file was bad, but the CSV export worked. If all three file types are bad, then the information below may possibly be of help.
There is a bugfix listed for version 16 of Machform that might relate to the problem you are seeing:
Bugfix: Exporting entries/form on server with zlib.output_compression turned on, result to a zipped content
If you haven't upgraded to version 16 yet, you might consider doing so. You might want to try contacting technical support for your web hosting service (or the IT department of your institution, etc.) to find out if this server option is actually turned on first.