Before adding images to PDF, you can choose the new page's size of PDF. It could be the standard PDF page size type(like A4), and also could keep the size from source image, which is from local file or stream.
And this tutorial is a guide to append images from the disk to an existed PDF document using vb.net code.
Imports PQScan.ImageToPDF Namespace AppendImageToPDF Class Program Private Shared Sub Main(args As String()) 'Create an instance of PQScan.ImageToPDF.PDFConverter object. Dim converter As New PDFConverter() 'Set PDF page layout, A4 is most widely used. converter.PageSizeType = PageSizeMode.A4 'Set input single tiff image from local files. Dim imgFiles As String() = New String(2) {"append1.jpg", "append2.jpeg", "append3.jpeg"} 'Set output pdf file name. Dim outFile As String = "merge-image-to-pdf.pdf" 'Append images to PDF. converter.AppendToPDF(imgFiles, outFile) End Sub End Class End Namespace