Editing PDF files in such a way is risky. They were never meant to be edited like that, and it can go wrong, like it did in your case...
The correct way is to go back to the original file used to create the PDF and edit it there. Alternatively, export the PDF to another format (Word document, InDesign layout, etc.), edit it, and then create a new PDF file from it.
Thank you for your answer, unfortunately we haven't got the original file because usually our customers send us this versions only, recentely we found out that these anomaly only occurs when there are cyrillic characters in the document.
I don't understand why even if we change just a date and we do not touch the text, this is altered nonetheless.
Are there any parameters to be set in order to use such files?
This usually happens when the file was created in a "non-conventional" way. There's nothing much you can do about it, unless you re-create the file (either using Acrobat by sending it to the PDF Printer, or by using a different application).
I think so too thank you