I’ve been able to fix it.
Here’s my manual procedure:
1. Convert the PDF as HTML and open it with MS Word.
2. After that, check the exact format of the “false” chapters, those which proceed from page numbers. With mouse right button, look for “select all text with same format”…and
3. DELETE IT
4. With this, I have obtained four page breaks that I’ve deleted replacing them (^p x4 character) by “nothing”.
Thanks for answers!