Are you using the Export PDF service through the Adobe Reader desktop application? Or online via https://cloud.acrobat.com/exportpdf
is it not exporting this one particular file or happening with all the document you are trying to export?
Make sure that the file that you are trying to export is not so large in size. It shouldn't be more than 100MB.
If it is desktop application, then you may try conversion online via- https://cloud.acrobat.com/exportpdf
And if you are working online, then try with a different web browser to see if this makes a difference or not.
I was able to log into the Cloud and complete it there. It just takes longer.
Thanks for sharing the update on this.