Hebrew text gets replaced by dots while i am working

I have several PDF files with English text.  I have been using Acrobat to edit the text , replacing the English with Hebrew.  I used OCR to make sure that all text in the file can be edited.

For 2 weeks now, every so often, all the text in the File suddenly gets replaced by dots.  it can be when i am in the middle of typing, or when i reopen a file which had been perfectly fine when i closed and saved it.
 

I 0ahve tried so many things, but nothing works.  I just worked for two hours on a file which I had started on Thursday. I saved my work every ten minutes, in case something would go wrong.  For 2 hours today, everything was fine, when suddenly, all the text was replaced by dots. I closed it without saving, but when i reopened it, the dots were there and not the text.