CODEWITHSUNDEEP
c#wkhtmltopdf
Sanketh. K. Jain
Sanketh. K. Jain

Reputation: 489

Unable to merge 2 PDFs using MemoryStream

I have a c# class that takes an HTML and converts it to PDF using wkhtmltopdf.
As you will see below, I am generating 3 PDFs - Landscape, Portrait, and combined of the two.

The properties object contains the html as a string, and the argument for landscape/portrait.

System.IO.MemoryStream PDF = new WkHtmlToPdfConverter().GetPdfStream(properties);
System.IO.FileStream file = new System.IO.FileStream("abc_landscape.pdf", System.IO.FileMode.Create);
PDF.Position = 0;

properties.IsHorizontalOrientation = false;
System.IO.MemoryStream PDF_portrait = new WkHtmlToPdfConverter().GetPdfStream(properties);
System.IO.FileStream file_portrait = new System.IO.FileStream("abc_portrait.pdf", System.IO.FileMode.Create);
PDF_portrait.Position = 0;

System.IO.MemoryStream finalStream = new System.IO.MemoryStream();
PDF.CopyTo(finalStream);
PDF_portrait.CopyTo(finalStream);
System.IO.FileStream file_combined = new System.IO.FileStream("abc_combined.pdf", System.IO.FileMode.Create);

try
{
    PDF.WriteTo(file);
    PDF.Flush();

    PDF_portrait.WriteTo(file_portrait);
    PDF_portrait.Flush();

    finalStream.WriteTo(file_combined);
    finalStream.Flush();
}
catch (Exception)
{
    throw;
}
finally
{
    PDF.Close();
    file.Close();

    PDF_portrait.Close();
    file_portrait.Close();

    finalStream.Close();
    file_combined.Close();
}

The PDFs "abc_landscape.pdf" and "abc_portrait.pdf" generate correctly, as expected, but the operation fails when I try to combine the two in a third pdf (abc_combined.pdf).

I am using MemoryStream to preform the merge, and at the time of debug, I can see that the finalStream.length is equal to the sum of the previous two PDFs. But when I try to open the PDF, I see the content of just 1 of the two PDFs.
The same can be seen below: PDF sizes

Additionally, when I try to close the "abc_combined.pdf", I am prompted to save it, which does not happen with the other 2 PDFs. Saving prompt

Below are a few things that I have tried out already, to no avail:

  1. Change CopyTo() to WriteTo()
  2. Merge the same PDF (either Landscape or Portrait one) with itself

    In case it is required, below is the elaboration of the GetPdfStream() method.
var htmlStream = new MemoryStream();
var writer = new StreamWriter(htmlStream);
writer.Write(htmlString);
writer.Flush();
htmlStream.Position = 0;
return htmlStream;

Process process = Process.Start(psi);
process.EnableRaisingEvents = true;
try
{
    process.Start();
    process.BeginErrorReadLine();

    var inputTask = Task.Run(() =>
    {
        htmlStream.CopyTo(process.StandardInput.BaseStream);
        process.StandardInput.Close();
    });

    // Copy the output to a memorystream
    MemoryStream pdf = new MemoryStream();
    var outputTask = Task.Run(() =>
    {
        process.StandardOutput.BaseStream.CopyTo(pdf);
    });

    Task.WaitAll(inputTask, outputTask);

    process.WaitForExit();

    // Reset memorystream read position
    pdf.Position = 0;

    return pdf;
}
catch (Exception ex)
{
    throw ex;
}
finally
{
    process.Dispose();
}

Upvotes: 13

Views: 2919

Answers (4)

Matthew Goulart
Matthew Goulart

Reputation: 3065

PDF files aren't just text and images. Behind the scenes there is a strict file format that describes things like PDF version, the objects contained in the file and where to find them.

In order to merge 2 PDFs you'll need to manipulate the streams.

First you'll need to conserve the header from only one of the files. This is pretty easy since it's just the first line.

Then you can write the body of the first page, and then the second.

Now the hard part, and likely the part that will convince you to use a library, is that you have to re-build the xref table. The xref table is a cross reference table that describes the content of the document and more importantly where to find each element. You'd have to calculate the byte offset of the second page, shift all of the elements in it's xref table by that much, and then add it's xref table to the first. You'll also need to ensure you create objects in the xref table for the page break.

Once that's done, you need to re-build the document trailer which tells an application where the various sections of the document are among other things.

See https://resources.infosecinstitute.com/pdf-file-format-basic-structure/

This is not trivial and you'll end up re-writing lots of code that already exists.

Upvotes: 4

Maytham Fahmi
Maytham Fahmi

Reputation: 33387

Merging pdf in C# or any other language is not straight forward with out using 3rd party library.

I assume your requirement for not using library is that most Free libraries, nuget packages has limitation or/and cost money for commercial use.

I have made research and found you an Open Source library called PdfClown with nuget package, it is also available for Java. It is Free with out limitation (donate if you like). The library has a lot of features. One such you can merge 2 or more documents to one document.

I supply my example that take a folder with multiple pdf files, merged it and save it to same or another folder. It is also possible to use MemoryStream, but I do not find it necessary in this case.

The code is self explaining, the key point here is using SerializationModeEnum.Incremental:

public static void MergePdf(string srcPath, string destFile)
{
    var list = Directory.GetFiles(Path.GetFullPath(srcPath));
    if (string.IsNullOrWhiteSpace(srcPath) || string.IsNullOrWhiteSpace(destFile) || list.Length <= 1)
        return;
    var files = list.Select(File.ReadAllBytes).ToList();
    using (var dest = new org.pdfclown.files.File(new org.pdfclown.bytes.Buffer(files[0])))
    {
        var document = dest.Document;
        var builder = new org.pdfclown.tools.PageManager(document);
        foreach (var file in files.Skip(1))
        {
            using (var src = new org.pdfclown.files.File(new org.pdfclown.bytes.Buffer(file)))
            { builder.Add(src.Document); }
        }

        dest.Save(destFile, SerializationModeEnum.Incremental);
    }
}

To test it

var srcPath = @"C:\temp\pdf\input";
var destFile = @"c:\temp\pdf\output\merged.pdf";
MergePdf(srcPath, destFile);

Input examples
PDF doc A and PDF doc B

PDF doc A and PDF doc B

Output example

Merged

Links to my research:

Disclaimer: A part of this answer is taken from my my personal web site https://itbackyard.com/merge-multiple-pdf-files-to-one-pdf-file-in-c/ with source code to github.

Upvotes: 10

Alexander Bruun
Alexander Bruun

Reputation: 277

This answer from Stack Overflow (Combine two (or more) PDF's) by Andrew Burns works for me: 

        using (PdfDocument one = PdfReader.Open("pdf 1.pdf", PdfDocumentOpenMode.Import))
        using (PdfDocument two = PdfReader.Open("pdf 2.pdf", PdfDocumentOpenMode.Import))
        using (PdfDocument outPdf = new PdfDocument())
        {
            CopyPages(one, outPdf);
            CopyPages(two, outPdf);

            outPdf.Save("file1and2.pdf");
        }

        void CopyPages(PdfDocument from, PdfDocument to)
        {
            for (int i = 0; i < from.PageCount; i++)
            {
                to.AddPage(from.Pages[i]);
            }
        }

Upvotes: 7

IOrlandoni
IOrlandoni

Reputation: 1828

That's not quite how PDFs work. PDFs are structured files in a specific format. You can't just append the bytes of one to the other and expect the result to be a valid document.

You're going to have to use a library that understands the format and can do the operation for you, or developing your own solution.

Upvotes: 4

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