Only first pdf file filled with fpdm can be opened - php

Question

this is my first question here so please excuse if this is incomprehensible.
I've got a little web tool on PHP for registering examinees to an examination. At the end I want to fill a PDF form with FPDM with data from the database in a foreach. The frist file which is created is perfect. The others are created but when I want to open with Acrobat or Chrome the loading is failed. I can fill every set of data separately but when I call the filling function twice or multiple times, the second file can not be opened.
MS Edge is able to open the currupt files but doesn't show the fonts correctly. So FPDM seems to fill all forms correctly but the files are somehow broken.
I'm searching for solutiones sincs days but no idea where is the problem. The code should be correct.
Can anybody help??
Here is the code:
require_once('libs/fpdm.php');
$statement = $pdo->prepare("SELECT * FROM `examinations` WHERE uid = :uid");
$result = $statement->execute(array('uid' => $_GET['prepareExamination']));
$examinationToPrepare = $statement->fetch();
$statement = $pdo->prepare("SELECT * FROM `examinees` WHERE examination = :examination");
$result = $statement->execute(array('examination' => $_GET['prepareExamination']));
$examinees = $statement->fetchAll();
$i = 1;
function mergePDF($template, $fields, $outfilename) {
header("Content-type:application/pdf");
header("Content-Disposition:attachment;filename=" . basename($outfilename) . "");
$pdf = new FPDM($template);
$pdf->Load($fields, true);
$pdf->Merge();
$pdf->Output($outfilename, "F")
}
foreach ($examinees as $examinee => $data) {
if ($examinationToPrepare['department'] == 1) {
$examiner1Name = utf8_encode(findByUid($_SESSION['examiners'], $examinationToPrepare['group' . strval(intval($data['subDepartment']) - 1) . 'FirstExaminer'], 'firstName')) . ' ' . utf8_encode(findByUid($_SESSION['examiners'], $examinationToPrepare['group' . strval(intval($data['subDepartment']) - 1) . 'FirstExaminer'], 'lastName'));
$examiner1Description = utf8_encode(findByUid($_SESSION['examiners'], $examinationToPrepare['group' . strval(intval($data['subDepartment']) - 1) . 'FirstExaminer'], 'graduation')) . "\n" . utf8_encode(findByUid($_SESSION['examiners'], $examinationToPrepare['group' . strval(intval($data['subDepartment']) - 1) . 'FirstExaminer'], 'function'));
$examiner2Name = utf8_encode(findByUid($_SESSION['examiners'], $examinationToPrepare['group' . strval(intval($data['subDepartment']) - 1) . 'SecondExaminer'], 'firstName')) . ' ' . utf8_encode(findByUid($_SESSION['examiners'], $examinationToPrepare['group' . strval(intval($data['subDepartment']) - 1) . 'SecondExaminer'], 'lastName'));
$examiner2Description = utf8_encode(findByUid($_SESSION['examiners'], $examinationToPrepare['group' . strval(intval($data['subDepartment']) - 1) . 'SecondExaminer'], 'graduation')) . "\n" . utf8_encode(findByUid($_SESSION['examiners'], $examinationToPrepare['group' . strval(intval($data['subDepartment']) - 1) . 'SecondExaminer'], 'function'));
} else {
$examiner1Name = utf8_encode(findByUid($_SESSION['examiners'], $examinationToPrepare['firstExaminer'], 'firstName')) . utf8_encode(findByUid($_SESSION['examiners'], $examinationToPrepare['firstExaminer'], 'lastName'));
$examiner1Description = utf8_encode(findByUid($_SESSION['examiners'], $examinationToPrepare['firstExaminer'], 'graduation')) . '\n' . utf8_encode(findByUid($_SESSION['examiners'], $examinationToPrepare['firstExaminer'], 'function'));
$examiner2Name = utf8_encode(findByUid($_SESSION['examiners'], $examinationToPrepare['secondExaminer'], 'firstName')) . utf8_encode(findByUid($_SESSION['examiners'], $examinationToPrepare['secondExaminer'], 'lastName'));
$examiner2Description = utf8_encode(findByUid($_SESSION['examiners'], $examinationToPrepare['secondExaminer'], 'graduation')) . '\n' . utf8_encode(findByUid($_SESSION['examiners'], $examinationToPrepare['secondExaminer'], 'function'));
}
$fields = array(
'name' => utf8_encode($data['firstName']) . ' ' . utf8_encode($data['lastName']),
'graduierung' => utf8_encode(findByUid($_SESSION['graduations'], $data['graduation'], 'name')) . ' ' . utf8_encode(findByUid($_SESSION['systems'], $data['system'], 'name')),
'ort' => utf8_encode($examinationToPrepare['location']),
'registrierung' => utf8_encode($examinationToPrepare['registration']),
'datum' => utf8_encode($examinationToPrepare['date']),
'pruefer1' => $examiner1Name,
'pruefer1-grad' => $examiner1Description,
'pruefer2' => $examiner2Name,
'pruefer2-grad' => $examiner2Description
);
if (!file_exists('documents/' . utf8_encode($examinationToPrepare['title']))) {
mkdir('documents/' . utf8_encode($examinationToPrepare['title']), 0777, true);
}
if ($examinationToPrepare['department'] == 1 && !file_exists('documents/' . $examinationToPrepare['title'] . '/' . findById($_SESSION['subDepartments'], $data['subDepartment'], 'name'))) {
mkdir('documents/' . $examinationToPrepare['title'] . '/' . findById($_SESSION['subDepartments'], $data['subDepartment'], 'name'), 0777, true);
}
mergePDF('pdf/Urkunde1.pdf', $fields, 'documents/' . $examinationToPrepare['title'] . '/' . ($examinationToPrepare['department'] == 1 ? findById($_SESSION['subDepartments'], $data['subDepartment'], 'name') . '/' : '') . utf8_encode($data['firstName']) . ' ' . utf8_encode($data['lastName']). '.pdf');
$i++;
}