Function find_header 

pub fn find_header<S: Source + ?Sized>(
    source: &S,
) -> Result<Header, HeaderError>

Finds the %PDF-M.m header in the first HEADER_SEARCH_WINDOW bytes.

The first %PDF- marker wins; if no version follows it, that is an error rather than a reason to keep looking, since a second header in the junk is no more trustworthy than the first.