I used fpdf several times to create php based pdf documents. The following example:
require('fpdf.php');
$pdf = new FPDF();
$pdf->AddFont('georgia', '', 'georgia.php');
$pdf->AddFont('georgia', 'B', 'georgiab.php');
$pdf->AddFont('georgia', 'I', 'georgiai.php');
$pdf->AddFont('freesans', '', 'freesans.php', true);
$pdf->SetFont('freesans', '', 12);
$pdf->SetTitle('My title');
$pdf->SetAuthor('My author');
$pdf->SetDisplayMode('fullpage', 'single');
$pdf->SetLeftMargin(20);
$pdf->SetRightMargin(20);
$pdf->AddPage();
$pdf->Cell(40,10,'Hello World!');
$pdf->Output();
You can learn these tutorials very quickly on the website itself.
EDIT : example of saving form data: (yes, very simple ...)
require('fpdf.php');
$pdf = new FPDF();
$pdf->AddPage();
foreach ($_POST as $key =>$data)
{
$pdf->Write(5, "$key: $data");
$pdf->Ln(10);
}
$pdf->Output($path_to_file . 'file.txt','F');
, fpdf, !

