The PdfViewer demo available in both C# and VB.NET shows you how to use it. 

This is code from the object reference in the documentation.  Once you add the decoder to the registered decoders you can open a PDF as you would any other image.

using Atalasoft.Imaging;
using Atalasoft.Imaging.Codec;
using Atalasoft.Imaging.Codec.Pdf;
...
PdfDecoder pdf = new PdfDecoder();
pdf.Resolution = 150;
RegisteredDecoders.Decoders.Add(pdf);
Workspace myWorkspace = new Workspace();
myWorkspace.Open("document.pdf");

Imports Atalasoft.Imaging
Imports Atalasoft.Imaging.Codec
Imports Atalasoft.Imaging.Codec.Pdf
...
Dim pdf As PdfDecoder = New PdfDecoder()
pdf.Resolution = 150
RegisteredDecoders.Decoders.Add(pdf)
Dim myWorkspace As Workspace = New Workspace()
myWorkspace.Open("document.pdf");