PDFsharp & MigraDoc Foundation

public void MergePDFsUsingPDFSharpWithSizeLimitNew(List<string> pdfFiles, string outputDirectory)
{
    const int maxFileSizeKB = 6000; // Maximum file size in KB
    var outputFileIndex = 1;
    var outputFileName = Path.Combine(outputDirectory, $"mergedUsingPdfSharp_{outputFileIndex}.pdf");

    _log.Info($"{nameof(PdfMergerService)}: MergePDFsUsingPDFSharpWithSizeLimitNew: Starting PDF merge...");

    var stopwatch = Stopwatch.StartNew();

    var outputDocument = new PdfDocument();

    foreach (var pdfFile in pdfFiles)
    {
        _log.Info($"{nameof(PdfMergerService)}: Processing file {pdfFile}");
        try
        {
            if (!File.Exists(pdfFile))
                throw new FileNotFoundException($"File not found: {pdfFile}");

            using var inputDocument = PdfReader.Open(pdfFile, PdfDocumentOpenMode.Import);
            for (int i = 0; i < inputDocument.PageCount; i++)
            {
                outputDocument.AddPage(inputDocument.Pages[i]);

                // Save to a temporary stream and check file size
                using var tempStream = new MemoryStream();
                outputDocument.Save(tempStream);
                var fileSizeKB = tempStream.Length / 1024; // Get size in KB

                // If the file size exceeds the limit, save the current PDF and start a new one
                if (fileSizeKB >= maxFileSizeKB)
                {
                    SaveCurrentOutput(outputDocument, outputDirectory, ref outputFileIndex);

                    // Start a new document
                    outputDocument = new PdfDocument();
                }
            }
        }
        catch (Exception ex)
        {
            _log.Error($"{nameof(PdfMergerService)}: Error processing {pdfFile}: {ex.Message}");
        }
    }

    // Save any remaining pages in the output document
    if (outputDocument.PageCount > 0)
    {
        SaveCurrentOutput(outputDocument, outputDirectory, ref outputFileIndex);
    }

    stopwatch.Stop();
    _log.Info($"{nameof(PdfMergerService)}: Merge completed in {stopwatch.ElapsedMilliseconds} ms.");
}

private void SaveCurrentOutput(PdfDocument outputDocument, string outputDirectory, ref int outputFileIndex)
{
    var outputFileName = Path.Combine(outputDirectory, $"mergedUsingPdfSharp_{outputFileIndex}.pdf");

    try
    {
        outputDocument.Save(outputFileName);
        _log.Info($"{nameof(PdfMergerService)}: Output saved to {outputFileName}");
    }
    catch (Exception ex)
    {
        _log.Error($"{nameof(PdfMergerService)}: Error saving merged PDF: {ex.Message}");
    }

    // Increment file index for the next output
    outputFileIndex++;
}