I've read links above and now I'm master of conversion
Unwrap factor together with delete empty paragraphs make it.
I had also pdf which in adobe reader were ok, but after conversion polish fonts were seriously garbled. I converted it in online service (all local converters like mobipocket and calibre converted all polish letters to the same letter) . Then I replace manually polish letters and printed it to pdf again, because text import doesn't have unwrap ability. Then one letter despite displaying everywhere ok were still garbled. I again copied pdf text to editor, replaced bad letter and saved as txt file. At last calibre converted it successfully...