Files sent through Adobe Send do not have expiration dates, unlike SendNow.
However, if you delete a file from your Acrobat.com account, or if you change its Sharing status, then recipients will not be able to download it. Please check the files in your account and make sure that they are still there, and are being Shared.
The easiest way to do this is to go to https://cloud.acrobat.com/sentfiles. Are the files that you sent still there? Note: ff they are shown in a grayed-out state, that indicates that they were 'Unshared.'
Do you realize how tedious this is when you send multiple files a day and they need to expire on a certain day for confidentiality purposes. I will not continue to use Adobe Send without an expiration option.