cannot open file
Hi there,
I work on this file last July 24, 2020 and the condition is still ok. In fact, I generate a pdf file and send it to author for review. Now when I'm about to open the file, the message pop-up:

I open file in order to implement some corrections given by the author. This file containing 650 pages and I'm desperate to use it again and save to another filename if really needed.
Please help, I need it badly.