El Capitan, update – Preview problem with visualizing texts

I have a huge issue with Preview after installing El Capitan.

I use to write on my PDF files directly with the text preview function.

Since I installed El Capitan, all the texts I wrote (I have plenty of files…) look like full colored squares. If I delete them, there is no text at all. Same result if I open it with Acrobat professional from a Mac or from a Windows, same result…

I hope some of you has the solution to this problem!
Thanks
Giulio