Why won't PDFs open (dropbox backup)

Hey there,

I've been using my Boox Note Air for over a year now for business and love it.

All this time I've assumed my notes are safely backed up to dropbox. 

Today I logged into dropbox and tried to open a PDF. It wouldn't open. Error. I downloaded and tried to open it with my Macbook. Nope. Error. The file shows as 'zero bytes' too, for some reason.

Is this normal? I mean, if this thing died I'd replace it as quickly as possible but I'd need access to these notes in the interim. Am I missing something?