public async Task<Stream> GetFileByStream(int id)
        {
            try
            {       
                PdfDocument outputPDFDocument = new PdfDocument();

                await foreach (var stream in getStreams())
                {
                    using PdfDocument inputPDFDocument = PdfReader.Open(stream, PdfDocumentOpenMode.Import);
                    outputPDFDocument.Version = inputPDFDocument.Version;

                    foreach (PdfPage page in inputPDFDocument.Pages)
                        outputPDFDocument.AddPage(page);
                }

                Stream pdfStream = new MemoryStream();
                    outputPDFDocument.Save(pdfStream);
                return pdfStream;
            }
            catch (Exception ex)
            {
                throw;
            }
                     
            async  IAsyncEnumerable<Stream> getStreams()
            {  
                using (SqlConnection connection = new SqlConnection(_connectionString))
                {
                    await connection.OpenAsync();
                    using (SqlCommand command = new SqlCommand(@" select pdffile.fileContent from pdffile
                    where insertedTaskId = @id", connection))
                    {
                        command.Parameters.AddWithValue("id", id);
                        using (SqlDataReader reader = await command.ExecuteReaderAsync(CommandBehavior.SequentialAccess))
                        {                            
                            while (await reader.ReadAsync())
                            {
                                if (!(await reader.IsDBNullAsync(0)))
                                {
                                    using (Stream data = reader.GetStream(0))
                                    {
                                        using var memoryStream = new MemoryStream();
                                        await data.CopyToAsync(memoryStream);
                                        yield return memoryStream;
                                    }
                                }
                            }
                        }
                    }
                }
            }
        } 

    public async Task<IActionResult> GetPdfByTaskFromStreamAsync([FromQuery] int id)
    {
       var result= await _PdfFileRepository.GetFileByStream(id);
       return new FileStreamResult(result, "application/octet-stream") { FileDownloadName = "file.pdf" };
    }

Stream data = reader.GetStream(0)

using PdfDocument inputPDFDocument = PdfReader.Open(stream, PdfDocumentOpenMode.Import);