:h help<tab>

gistfile1.txt

function! Hoge() abort
    let lines = []
    for row in range(1, 999)
        let line = ''
        for col in range(1, &columns)
            let c = screenchar(row, col)
            if 0 < c
                let line .= nr2char(c)
            endif
        endfor
        if !empty(line)
        let lines += [line]
    endif
    endfor
    return writefile(lines, 'temp.txt')
endfunction
cnoremap <expr><tab> Hoge()
call feedkeys(":h help\<C-d>\<tab>\<C-u> echo join(readfile('temp.txt'), \"\\n\")\<cr>")