So sorry for the snag, @Fang!
We can reproduce the same behavior indeed and it’s on our list to fix. We’ll get to it!
In the meantime, you should be able to export it without any issue using Mockups 3 for Desktop.
All you need to do is to download your project from Balsamiq Cloud, open it in Mockups 3 for Desktop and export the PDF from there.
Sorry for the extra steps but I figured it might help as a workaround while we work on fixing the issue in Balsamiq Cloud.
Hope this helps! Anything else, we’re here.