You open the file:
if((mystream = sfd.OpenFile()) != null)
You close the file:
But all that does is delete any existing file of that name, and create a new, empty file.
Adobe won't be able to read the empty file as it contains no PDF data. So rightly, it complains.
Somewhere, you have PDF data that you want in the file - at some point to need to pass it to the mystream.Write method, probably using this overload: Stream.Write Method (System.IO) | Microsoft Docs
We can't be more specific, we have no idea where your data is or how it is stored!