It's not exactly Open Source, but Aspose has a couple products which can do that,
Aspose.Pdf.Kit
Aspose.Pdf.Kit is a non-graphical PDF® document manipulation component that enables both .NET and Java developers to manage existing PDF files as well as manage form fields embedded within PDF files. Aspose.Pdf is perfect for creating new PDF files; however, developers often need to edit already existing PDF documents. Aspose.Pdf.Kit allows them to do just that. Aspose.Pdf.Kit allows developers to create powerful applications for merging data directly into PDF documents as well as for updating and managing PDF documents. Aspose.Pdf.Kit is a wonderful product and works great with the rest of our PDF products.
and Aspose.pdf
Aspose.Pdf is a non-graphical PDF® document reporting component that enables either .NET or Java applications to create PDF documents from scratch without utilizing Adobe Acrobat®. Aspose.Pdf is very affordably priced and offers a wealth of strong features including: compression, tables, graphs, images, hyperlinks, security and custom fonts. Aspose.Pdf supports the creation of PDF files through API, XML templates and XSL-FO files. Aspose.Pdf is very easy to use and is provided with 14 fully featured demos written in both C# and Visual Basic.
Check out the API and demos. You can download a DLL for free to try it out. I've used both before and they work out great.
There's also iTextSharp which is a C# port of iText, a Java PDF converter. I've heard some people try it with mixed results.
