This error can occur if the HiQPdf.dep resource file there is not in the same folder with HiQPdf.dll assembly at runtime. When you reference the HiQPdf.dll in your application in Visual Studio and you build that application, HiQPdf.dll and HiQPdf.dep will be automatically copied in the Bin folder of the application. Moreover, the HiQPdf.dep file will be automatically copied near the HiQPdf.dll when you add the assembly to GAC. This means that in general the two files will stay together, but there might be situations when this does not happen for example when you manually copy the files on a server and you forget to copy the HiQPdf.dep or if the assembly is automatically copied in a temporary location at runtime by a shadow copy mechanism which doesn't take into account the assembly resources.