为什么我的 PDF 文件大小在拆分和合并后会增加？ （使用 PDFSharp c#）

Question

I am basically splitting a PDF document into multiple documents containing one page each. After splitting I perform some operations and the merge the documents back to a single PDF. I am using PDFsharp in c# to do this. Now the problem I am facing is that when I split the document and then add them back, the file size increases from 1.96Mbs to 12.2Mbs. Now after thoroughly testing, I have pointed out that the problem lies not in the operations which I performing after splitting but in the actual splitting and merging of PDF documents. The following are my functions which I have created.

 public static List<Stream> SplitPdf(Stream PdfDoc)
    {
        System.Text.Encoding.RegisterProvider(System.Text.CodePagesEncodingProvider.Instance);
        List<Stream> outputStreamList = new List<Stream>();
        PdfSharp.Pdf.PdfDocument inputDocument = PdfReader.Open(PdfDoc, PdfDocumentOpenMode.Import);

        for (int idx = 0; idx < inputDocument.PageCount; idx++)
        {
            PdfSharp.Pdf.PdfDocument outputDocument = new PdfSharp.Pdf.PdfDocument();
            outputDocument.Version = inputDocument.Version;
            outputDocument.Info.Title =
              String.Format("Page {0} of {1}", idx + 1, inputDocument.Info.Title);
            outputDocument.Info.Creator = inputDocument.Info.Creator;

            outputDocument.AddPage(inputDocument.Pages[idx]);
            MemoryStream stream = new MemoryStream();
            outputDocument.Save(stream);
            outputStreamList.Add(stream);
        }
        return outputStreamList;
    }

 public static Stream MergePdfs(List<Stream> PdfFiles)
    {
        System.Text.Encoding.RegisterProvider(System.Text.CodePagesEncodingProvider.Instance);
        PdfSharp.Pdf.PdfDocument outputPDFDocument = new PdfSharp.Pdf.PdfDocument();
        foreach (Stream pdfFile in PdfFiles)
        {
            PdfSharp.Pdf.PdfDocument inputPDFDocument = PdfReader.Open(pdfFile, PdfDocumentOpenMode.Import);
            outputPDFDocument.Version = inputPDFDocument.Version;
            foreach (PdfSharp.Pdf.PdfPage page in inputPDFDocument.Pages)
            {
                outputPDFDocument.AddPage(page);
            }
        }
        Stream compiledPdfStream = new MemoryStream();
        outputPDFDocument.Save(compiledPdfStream);
        return compiledPdfStream;
    }

The question which I have is:

1. Why am I getting this behaviour?
2. Is there a solution where I can perform split and merge and then get the file of same size? (Can be of any open-source c# library)

Answer 1

Replying to question 1:
When splitting the files, every file will contain all resources required by the pages it contains.

When merging with PDFsharp again, resources will not be merged and the final document may contain duplicated resources (fonts, images), thus leading to larger files.
This is by design.