We are sorry for the delay in response to your query.
This was an intermittent issue reported with the Document Cloud website. However, the issue should now been resolved with the new interface of of DC web. But if you are still experiencing the same problem, then you may try couple of network troubleshooting suggested in the similar discussion here- Re: Conversion Failure
If you are working with Adobe Reader DC desktop application, then you may check whether the latest version has been installed or not?