Ensure that all PDF content is transferred to the output file while merging

Question

I'm trying to merge multiple PDF files using the tool Setasign/FPDI along with their parser FPDI PDF-Parser.

Some PDF files can have drawings like signatures before they are merged. The problem is that after the merging, those drawings are gone.

I'm looking for a solution to ensure that all PDF content is transferred to the final merged output file.

Any help is greatly appreciated.

    private function mergeFiles()
    {
        $fpdi = new Fpdi();

        foreach ($files as $file) {
            $this->mergePDFFile($fpdi, $file->getFilePath());
        }

        $fpdi->Output($outputFilePath, 'F');
    }


    private function mergePDFFile(Fpdi $pdf, $filePath)
    {
        $pageCount = $pdf->setSourceFile($filePath);

        for ($pageNo = 1; $pageNo <= $pageCount; $pageNo++) {
            $templateId = $pdf->importPage($pageNo);
            $size       = $pdf->getTemplateSize($templateId);
            $pdf->AddPage($size['orientation']);
            $pdf->useTemplate($templateId, null, null, null, null, true);
        }
    }

The original files before merging

The merged files with the unexptected result