What is the size of the pdf files that you are trying to send?
Are you able to select and send other file types?
Try using a different browser and check.
Try if you can send the file via SendNow instead of Send. Try this as a temporary circumvention only.
[topic moved to Adobe Send forum]
My first attempt had been from the Send box on the Adobe Reader screen. That had failed. Once I got properly registered into Adobe Send I was able to send the same group of files with no problem although I noticed that the embedded hyperlinks used to connect the files didn't work for the Send recipient. Hyperlinks are not a deal breaker as long as I can send the files as a group.