Is there any way to join two pdfs together?

[Finneah]

i have downloaded a PDF file and saved them as a file. Then i created another PDf File. When I try to make one out of these two files with writeStream(filePath,'base64',true) or with appendFile(filepath,data,'base64') I always have only the content of the second file.

Lib Version: 0.12.0

       RNFetchBlob.config({
                fileCache: true,
                path: filePath
            })
                .fetch(
                    'POST',
                    'https://app.useanvil.com/api/v1/fill/rTtjlIZnny79WW1WofOO.pdf',
                    {
                        Authorization:
                            '...',
                        'Content-Type': 'multipart/form-data'
                    },
                    [
                        {name: 'data', data: JSON.stringify(data)}
                    ]
                )
                .then(async (res) => {
                    let status = res.info().status;
                    if (status == 200) {
                        var pdf = new PDF_HA();
                        var html = pdf._getPDF(this.state);
                        PDF.fromHTML(html, '').then((protocolData) => {
                            RNFetchBlob.fs
                                .writeStream(filePath, 'base64', true)
                                .then((stream) => {
                                    stream.write(protocolData);
                                    RNFetchBlob.fs
                                        .readFile(filePath, 'base64')
                                        .then((allData) => {
                                            this.setState({
                                                filePath: filePath,
                                                base64: allData
                                            });
                                            return stream.close();
                                        });
                                });
                        });
                    }
                })
                .catch((err) => {
                    error_handler._handleDefaultError(
                        err
                    );
                });