I assume you are working with Adobe Reader DC desktop application, where you have added your "Dropbox" account to access the files?
What happen when you are trying to access the Dropbox files? Does it gives any error or is it freezing?
You may check the following steps to see if that helps-
1- Login to the Dropbox account online and see whether its accessible or not.
2- Check for Adobe Reader DC (19.0008.20074) latest version whether its installed or not. Open Reader>Help>Check for update.
3- Remove the Dropbox account from Adobe Reader and reconfigure it. Access your Dropbox files in Acrobat/Reader