I have the answer for my question about changing the style of lines. I found the solution in FPDF website (code from yukihiro_o converted by me).
Add the following code to the library:
Interface:
Procedure SetDash(black: double = 0.0; white:double = 0.0);
Code:
Procedure TJPFpdf.SetDash(black: double; white:double);
Var s:string;
begin
if(black<>0) then
s:=format('[%.3F %.3F] 0 d',[black*pgk,white*pgk],TPDFFormatSetings)
else
s:='[] 0 d';
_out(s);
end;
example of use:
SetDash(0.1,0.2);
Draw the next lines with small dash.