Sometimes you are trying to create a PDF from many images and you need to process them before they are saved.  PdfEncoder is written so that it can keep only one Image at a time in memory as it is building up the PDF Stream.  This article tells you how you can hook into the process to change the image if you need to.

To be able to process images you need to use the variant of PdfEncoder.Save() that uses an ImageSource.  ImageSource is designed for you to subclass, and it gives you the hooks you need to do this.  Here is the main function to read in list of files and put all of their frames into a PDF using a custom ImageSource and a SetEncoderCompression event to set the compression.

C#

public void SaveToPdf(string[] imgs, string outputFile)
{
    ImageSource pdfpages = new ProcessImageFSImageSource(imgs, true); 
    using (FileStream fs = new FileStream(outputFile, FileMode.Create, FileAccess.Write)) 
    { 
        PdfEncoder pdfEncoder = new PdfEncoder();

        // this lets me set the compression type of the image
        pdfEncoder.SetEncoderCompression +=
                  new EncoderCompressionEventHandler(pdf_SetEncoderCompression);

        pdfEncoder.Save(fs, pdfpages, null);
    }
}

ProcessImageFSImageSource is a new ImageSource -- in this case, I'm going to base it on FileSystemImageSource.  Here it is

public class ProcessImageFSImageSource : FileSystemImageSource
{
    public ProcessImageFSImageSource(string[] files, bool doAllFrames) :
          base(files, doAllFrames)
    {
    }

    // override the function that is called to get images and change them if necessary 
    protected override ImageSourceNode LowLevelAcquire(int index)
    {
        ImageSourceNode node = base.LowLevelAcquire(index);
        if (node != null && node.Image != null)
        {
            // here you can alter node.Image to be a new 
            // image using any DotImage function
        }
        return node;
    }
}

And here is the event handler we added to SetEncoderCompression

// (called by PdfEncoder's SetEncoderCompression event)
// this lets me set the compression type of the image
void pdf_SetEncoderCompression(object sender, EncoderCompressionEventArgs e)
{
    // e will have information about the Image that 
    // you can use to decide the right compression
    // here we just always use CcittGroup4 (which is only for 1bit images)
    e.Compression =
        new PdfCodecCompression(PdfCompressionType.CcittGroup4);
}

VB.NET

Public Class ProcessImageFSImageSource
    Inherits Atalasoft.Imaging.FileSystemImageSource

    Public Sub New(ByVal files As String(), ByVal doAllFrames As Boolean)
        MyBase.New(files, doAllFrames)
    End Sub

    ' override the function that is called to get images and change them if necessary
    Protected Overloads Overrides Function LowLevelAcquire(ByVal index As Integer) As Atalasoft.Imaging.ImageSourceNode
        Dim node As Atalasoft.Imaging.ImageSourceNode = MyBase.LowLevelAcquire(index)
        If node IsNot Nothing AndAlso node.Image IsNot Nothing Then
            ' here you can alter node.Image to be a new
            ' image using any DotImage function
        End If
        Return node
    End Function
End Class

Public Sub SaveToPdf(ByVal imgs As String(), ByVal outputFile As String)
    Dim pdfpages As Atalasoft.Imaging.ImageSource = New ProcessImageFSImageSource(imgs, True)
    Using fs As New System.IO.FileStream(outputFile, System.IO.FileMode.Create, System.IO.FileAccess.Write)
        Dim pdfEncoder As New Atalasoft.Imaging.Codec.Pdf.PdfEncoder()
        ' this lets me set the compression type of the image
        AddHandler pdfEncoder.SetEncoderCompression, _
            AddressOf pdf_SetEncoderCompression
        pdfEncoder.Save(fs, pdfpages, Nothing)
    End Using
End Sub

Private Sub pdf_SetEncoderCompression(ByVal sender As Object, ByVal e As Atalasoft.Imaging.Codec.EncoderCompressionEventArgs)
    ' e will have information about the Image that
    ' you can use to decide the right compression
    ' here we just always use CcittGroup4 (which is only for 1bit images)
    e.Compression = New Atalasoft.Imaging.Codec.Pdf.PdfCodecCompression(Atalasoft.Imaging.Codec.Pdf.PdfCompressionType.CcittGroup4)
End Sub