Use \texorpdfstring; the first argument can contain (La)TeX code and the second one (text only), will be used for the bookmarks:

\section[this is a very long title I want to break manually]{\texorpdfstring{this is a very long title I\\ want to break manually}{this is a very long title I want to break manually}}

If a ToC is to be produced and the break line is not desired there, a better option is to to use the optional argument of \section (this will be used for the headers, the ToC and the bookmarks):

\section[this is a very long title I want to break manually]{this is a very long title I\\ want to break manually}

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