How to restore PDF markups?

Hi there, 

I was reading a PDF document on iPad and made many comments and highlights using the Acrobat Reader app. I left the document open and locked the iPad.. but upon unlocking it, I ecountered a message reading "System error". I clicked "Ok" then had the document closed automatically. After I opened the document back, I found out that all markups disappeared. 

The file is a local one (not on iCloud).

Would appreciate any clues to restore these markups.