Best file format for transfer of EHR data

Question

I am working on a clinical trial where we have several sites sending us EHR data. The sites are currently sending the data in excel files. I have a feeling someone's opening the files because 3 of the files have 64,999 rows exactly, and excel 2007 cuts off at 65,000.

I am working in python, but I am trying to prevent the people at the local sites from opening the files in excel.

What's the best format for the files to be sent to me such that they can't be opened and cut off in excel? csv unfortunately can also be opened in excel.