I was working on a email invoicing application a few days ago, and I needed a way to consolidate multiple PDF invoices (approximately ~50-200 invoice p/week). I wanted something that was free and worked with C#, I found PdfSharp and after looking at the demo’s I came up with a quick solution. The process is really simple and can be summarized as (Create a new empty PDF, open each existing PDF and copy their pages into the new PDF). Here is a code snippet of how I combined the PDF files, using a DataReader that has the source URL of each invoice:

	// ********************************************************************************************* //
	// This is just a simplified excerpt, I was looping over all the invoices from the previous week
	// by customer, and aggregating their PDF invoices into a single document.  In addition to the
	// generating the PDF, I used the DataReader to also generate a CSV file of their invoice details.
	// ********************************************************************************************* //

	// Combined PDF
	var customerPdf = new PdfDocument();

	// Hold copy of last document
	string lastDocumentUrl = string.Empty;

	while (dr.Read())
		// Get current document
		string documentUrl = dr["InvoiceURL"].ToString();

		// Check if last document added is the same as current document
		if (lastDocumentUrl != documentUrl)
			// Set the last document equal to the current document
			lastDocumentUrl = documentUrl;

			// Get the current file (e.g. \\\\\.pdf)
			string filePath = documentUrl;

			// Read in the existing document
			PdfDocument inputDocument = PdfReader.Open(filePath, PdfDocumentOpenMode.Import);

			// Get # of pages
			var count = inputDocument.PageCount;

			// Add each page to "Combined PDF"
			for (int idx = 0; idx < count; idx++)
				PdfPage page = inputDocument.Pages[idx];