Asciidoctor PDF has built in logic for breaking before chapter titles and before and after part titles. As Alexander said, these breaks can be controlled using the heading-chapter-break-before, heading-part-break-before, and heading-part-break-after keys in the theme. Those are the only controls provided, atm. The value "auto" means to let it fall where it will fall...no added breaking logic.
> I got a document an i will decide by my own where a page break should be inserted.
Then I might recommend using the article doctype instead of the book doctype. The article doctype just lets the document flow. But it can be emulated by setting the keys mentioned above to "auto".