Project Setup

The logic to save the first page of a PDF to an image file is straight-forward:

using (PdfiumViewer.PdfDocument pdfDocument = PdfiumViewer.PdfDocument.Load(<PdfFilePath>))
{
    System.Drawing.Image bitmapImage = pdfDocument.Render(0, 200, 200, true);
    bitmapImage.Save(<ImageFileName>, System.Drawing.Imaging.ImageFormat.Jpeg);
}

In example above we're saving the first page (index 0) of the PDF (PdfFilePath) to the target file (ImageFileName) at 200 dpi vertical and horizontal resoution, rendering for printing. This example doesn't size the output image. It looks to be a clean image, but its size is small. It would likely need to be zoomed ~250% to be viewed normally. The example below addresses this, but realize the increased size means increased file size. You may find that the example above meets your needs.

In our case, we want this functionality contained in a .Net Framework library project, so the library project is setup as defined uder the "Project Setup" link above. The application project includes a reference to the library project and appropriate Nuget package files are automatically included in the compiled app structure, there's no need to add the Nuget packages to the consuming application project.

We'll be working with image files in the application and don't want to install/maintain System.Windows.Drawing.Common in application projects just to use the library, so we use an image format enum defined in the library class. The members match those in the System.Drawing.Imaging.ImageFormat enum. You could just as easily add System.Windows.Drawing.Common to the application project and use System.Drawing.Imaging.ImageFormat.

Our library project method:

public static List<string> SaveToImages(System.IO.FileInfo SourcePdf, string PageSpecs, string OutputFileBase, PSI.PDF.Procs.IMAGEFORMAT ImgFormat, bool Overwrite)
{
	int dpiX = 200;
	int dpiY = 200;
	int imgHeight;
	int imgWidth;
	string file;
	List<string> savedImages = new List<string>();
	IList<System.Drawing.SizeF> pageSizes;
	List<int> savePages = null;
	ImageFormatSpecs ifs = null;
	System.Drawing.Image bitmapImage;

	if (SourcePdf == null || !System.IO.File.Exists(SourcePdf.FullName))
		throw new ArgumentException("\"SourcePdf\" must be defined and exist");

	if (OutputFileBase == null || !System.IO.Directory.Exists(System.IO.Path.GetDirectoryName(OutputFileBase)))
		throw new ArgumentException("\"OutputFileBase\" must be defined and the target folder must exist");

	if (PageSpecs == null || PageSpecs.Trim().Length == 0)
		throw new ArgumentException("\"PageSpecs\" must be defined");

	ifs = PSI.PDF.Procs.GetImageFormatSpecs(ImgFormat);

	using (PdfiumViewer.PdfDocument pdfDocument = PdfiumViewer.PdfDocument.Load(SourcePdf.FullName))
	{
		//Generate a list of page numbers to save from the PageSpecs parameter
		savePages = PSI.PDF.Procs.PageSpecsToList(PageSpecs, pdfDocument.PageCount);
		if (savePages == null || savePages.Count == 0)
			throw new ArgumentException("PageSpecs resulted in no pages to save");

		//Make sure we can process all specified pages
		foreach (int i in savePages)
		{
			file = OutputFileBase + "-Page_" + i.ToString() + "." + ifs.Extension;
			if (pdfDocument.PageCount < i)
				throw new ArgumentException("Invalid page specified: " + i.ToString() + " | PDF contains " + pdfDocument.PageCount.ToString() + " pages");
			if (System.IO.File.Exists(file) && !Overwrite)
				throw new ArgumentException("Target image exists, overwrite not identified: " + file);
		}

		//Get the pages sizes array
		pageSizes = (IList<System.Drawing.SizeF>)pdfDocument.PageSizes;

		//Process the specified pages
		foreach (int i in savePages)
		{
			file = OutputFileBase + "-Page_" + i.ToString() + "." + ifs.Extension;
			imgWidth = (int)((pageSizes[i - 1].Width / 72) * dpiX);
			imgHeight = (int)((pageSizes[i - 1].Height / 72) * dpiY);
			bitmapImage = pdfDocument.Render(i-1, imgWidth, imgHeight, dpiX, dpiY, true);
			bitmapImage.Save(file, ifs.ImageFmt);
			savedImages.Add(file);
		}
	}
	return savedImages;
}