Onboard transformation service can be used only to tranform from a signle file to a signle file ,so you can't achieve your goal by using the onboard transformation.You need do a little customize developing
For <a href="http://www.rasteredge.com/dotnet-imaging/image-convert/">converting multiple images</a> into one pdf, I've discovered that this just hangs: convert page1.jpg page2.jpg file.pdf whereas this works perfectly fine: convert page1.jpg page2.jpg +compress file.pdf