Error when adding a new page from a PDF Using FPDF

Good morning, I'm trying to make an algorithm that takes several images and several PDFs and converts them into a single PDF, making it easier to upload files to my system.

I have the following codes:

$pdf = new FPDF();
    $Fpdfi = new setasign\Fpdi\Fpdi();
    foreach ($uploaded_files as $file) {
        ob_start();
        $extension = pathinfo($file, PATHINFO_EXTENSION);
        if (in_array($extension, array('jpg', 'jpeg', 'png', 'gif'))) {
            $pdf->AddPage();
            $pdf->Image($file);
        } elseif ($extension == 'pdf') {
            $pdf->AddPage();
            $pdf->SetFont('Arial', '', 12);
            $pdf->MultiCell(0, 5, "PDF file: $file");
            $pdf->AddPage();
            $Fpdfi->setSourceFile($file);
            $tplIdx = $Fpdfi->importPage(1);
            $Fpdfi->useTemplate($tplIdx, 10, 10, 90);         }
        ob_end_flush();
    }

    $output_file = $uploads_dir . '/result.pdf';
    $pdf->Output('F', $output_file);

When it gets to the line $Fpdfi->useTemplate($tplIdx, 10, 10, 90); it returns the following error:

Uncaught Exception: FPDF error: No page has been added yet in
C:...\plugins\fpdf\fpdf.php:273
Stack trace: #0 C:...\plugins\fpdf\fpdf.php(1464):
FPDF->Error('No page has bee...') #1
C:\xampp...\plugins\Fpdi\FpdfTplTrait.php(467):
FPDF->_out('q 0 J 1 w 0 j 0...') #2
C:...\plugins\Fpdi\FpdiTrait.php(441):
setasign\Fpdi\FpdfTpl->_out('q 0 J 1 w 0 j 0...') #3
C:...\plugins\Fpdi\Fpdi.php(59):
setasign\Fpdi\Fpdi->useImportedPage('C:\xampp\htdocs...', 10, 10, 90, NULL, false) #4
C:...\themes\easy\testeapi\teste_pdf.php(56):
setasign\Fpdi\Fpdi->useTemplate('C:\xampp\htdocs...', 10, 10, 90) #5
C:...\wp-includes\template-loader.php(74): include('C:\...\plugins\fpdf\fpdf.php on line 273

OBS: this only happens when I try to attach the PDF, if I only use Images, it works very well

I did several tests, and several searches including using ChatGPT, but I still couldn't reach the expected. the idea is to upload x files (PDF and IMAGE), and transform them into a single PDF.

You are using two class instances - that's wrong.

Just use the one for FPDI:

$pdf = new new setasign\Fpdi\Fpdi();
foreach ($uploaded_files as $file) {
    $extension = pathinfo($file, PATHINFO_EXTENSION);
    
    if (in_array($extension, array('jpg', 'jpeg', 'png', 'gif'))) {
        $pdf->AddPage();
        $pdf->Image($file);
    } elseif ($extension == 'pdf') {
        $pdf->AddPage();        
        $pdf->SetFont('Arial', '', 12);
        $pdf->MultiCell(0, 5, "PDF file: $file");
        $pdf->setSourceFile($file);
        $tplIdx = $pdf->importPage(1);
        $pdf->useTemplate($tplIdx, 10, 10, 90);     
    }
}

$output_file = $uploads_dir . '/result.pdf';
$pdf->Output('F', $output_file);

I removed the code which is useless IMHO. You also should only execute this script only if there are any files (image or pdf) uploaded. Otherwise you will end in the same error as you initially had.