Text disappeared after I closed the file

Hi, 

I have worked several days on a PDF document with Adobe Deader DC without any problems. But today, I saved my document, closed it and sent it, and the file I have sent has half the characters missing. I don't understand how it happened since I last saved the document with everything in place.

All my comments and highlight appears just right, but the entire text is like the images below. I even have several text boxes entirely blank. Surprizingly though, if I select a text section and cut-paste it, the missing characters of this section come back.

This problem has already happened with another document. It got back to normal after a few minutes of me panicking, I'm not sure how it was solved. I have tried converting it to PostScript, printing it into another PDF, exporting it in Word, nothing seems to work to retrieve the entire file. 

If possible, I would really like to find a solution that wouldn't require selecting every single text box and adjusting the entire layout again.  

Thanks in advance for your help !